That's the point the Romans(beast) isn't suppose to exist after Revelation 19:21 but they existed for hundreds of years after ad70 and split into the eastern and western empires and onward still after that.
I have an answer for this but you aren't going to like it. Your first impulse would be to disregard my explanation without giving it serious thought. This short question would take pages to fully explain but I'll try to make it as short as possible. Revelation is full of metaphors, one word actually means something else and John borrows symbols from the OT prophets. In a lot of cases, this symbolism has both spiritual and literal meanings. Yes, the Beast was the Roman empire but the beast was also it's leaders, "Caesar is Rome and Rome is Caesar" they used to say. Before I explain Rev 19 we must first look at Rev 9 where Titus is introduced because Rev 19 is the reverse of Rev 9.
11 And they had as king over them the angel of the bottomless pit (or Abyss), whose name in Hebrew is Abaddon, but in Greek he has the name Apollyon.
I believe Titus was the "angel of the bottomless pit" mentioned here. The "Abyss" or "bottomless pit" or "Lake of Fire" all mean the same thing. In the spiritual realm, we know this place as Sheol or Hades, the underworld realm of the spiritually dead - a place of torment and separation from God. Apollyon is a play on words for “Apollo” (Apollon in Greek) and “destroyer” a perfect description of Titus, the general of
Legio XV Apollonaris (the 15th Apollonian Legion) who destroyed Jerusalem and its Temple in A.D. 70 after tormenting them for five months with his scorpion dark-throwing machines. Apollyon is called “the angel of the Abyss.” (Revelation 9:11) Here Titus as Apollyon is said to rise out of the Abyss (the lake of fire) when he arrives at Jerusalem in A.D. 70 with his army.
Notice Titus was ALIVE before he arrived in Jerusalem to besiege the city. In other words, Titus was ALIVE when he was previously in the Abyss (lake of fire). So if Titus was PHYSICALLY ALIVE before he ROSE out of the Abyss (lake of fire) obviously he would still be ALIVE when he was cast back into Abyss (the lake of fire) just as is stated in Rev 19:20:
“The two of them were thrown ALIVE into the fiery lake of burning sulfur.”
In the OT and throughout Revelation the "earth" symbolizes a specific land, usually Israel while the sea symbolizes a foreign nation, in this case, Rome. Additionally,
when the Jews were in the holy land they were "alive," but when they were taken into captivity, they were "dead." Thus when they were restored to their land, they were "resurrected," made alive again. We see this symbolism in Ezekiel 37, Valley of Dry Bones. Here Ezekiel uses resurrection motif to describe Israel's return following Babylonian captivity. The same symbolism is denoted in Isaiah 28:14-18. In Isaiah 28:14-18 the leaders of Judah are said to have made a “covenant with death” and an “agreement with Sheol.” Like the Abyss/lake of fire, Sheol is another name for the realm of the dead. This “covenant with death” and “agreement with Sheol” poetically refers to the treaty Ahaz, king of Judah, made with Assyria to destroy the king of Israel, Pekah, and the king of Syria, Rezin. T
hus in Isaiah 28:14-18 foreign nations like Assyria are called “Sheol” and “death.” I further believe Titus was the rider of the pale horse of the fourth seal where his name is Death and Hades, but let's not digress.
In Ezek 37, the meaning of the vision is explained to Ezekiel where this resurrection is a metaphor for the return of the Jews from exile:
“Therefore prophesy and say to them: ‘This is what the Sovereign Lord says: My people, I am going to open your graves and bring you up from them; I will bring you back to the land of Israel.” (Ez 37:12.) If the return of the Jews from foreign nations is an symbolic of the resurrection of the dead, what does that imply when one leaves Israel? This departure out of Israel to foreign nations according to Ezekiel 37 represents the metaphorical death of the Jews. The same meaning is found in many other verses in the Bible and even appears in the Midrash Psalm 116:3 where “the power of the netherworld [Sheol/Abyss]” of Hosea 13:14 is interpreted as “the kingdoms” of the world. The same meaning appears implied in Rev 19:20 where the beast and false prophet are thrown into the Abyss (lake of fire) in A.D. 70. Here the departure of the beast to Rome, the Abyss/death/lake of fire, is symbolically denoting the death of the beast, as is often the case in the Bible and Jewish tradition.
The fact that Titus and Vespasian “were thrown alive into the fiery lake of burning sulfur” is echoed in 2 Thessalonians 2:8 which literally reads,
“And then the Lawless One will be revealed, whom the Lord Jesus will take away with the breath of his mouth and do away with by the splendor of his coming.” Echoing this interpretation of Revelation 19:20 Titus is taken away and done away with in 2 Thessalonians 2:8 having left Jerusalem for Rome, the Abyss, at the fall of the city in A.D. 70. Thus Revelation 19:20 and 2 Thessalonians 2:8 are both fulfilled in Titus’ departure to Rome, the Abyss, in A.D. 70 after Jerusalem fell.
You assume, as many do, that the casting of the beast and false prophet into the lake of fire must be fulfilled in the geopolitical fall of a kingdom. The lake of fire is the Abyss (Rev 9). The kingdom of the beast never fell geopolitically when it received its fatal wound (Rev 13:3) and was originally cast into the Abyss (Rev 9; 11:7; 17:8) and the outer darkness of death (Rev 16:10). If the beast never fell geopolitically when it was originally cast into the Abyss/lake of fire, why would it suddenly fall when it is cast into the Abyss/lake of fire again in Rev 19:20?
In fact, the kingdom of the beast could not fall even at the death of the beast in light of Daniel 7:11-12:
“I kept looking until the beast was slain and its body destroyed and thrown into the blazing fire. (The other beasts had been stripped of their authority, but were allowed to LIVE for a period of time.)” The “other beasts” of Dan 7:12 are Babylon, Medo-Persia and Greece which all fell long before the fourth beast came to power. Notice that these three empires “were allowed to
live” after they “had been stripped of their authority.” In other words, Babylon, Medo-Persia and Greece were still
alive after they had fallen geopolitically. If “death” symbolizes the geopolitical fall of a kingdom, why are these three empires explicitly said to live after they had fallen?
Spiritually speaking today, both Vespasian and Titus are most likely in the spiritual lake of fire as we speak where they would burn forever along with those who took the mark - the Jewish religious leaders who killed Christ.
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