So, this is a bit complicated of a plot, but if I can get it to work, I really want to run it, and my players are very interested if I can make it work. I have limited experience with homebrew, so I'm struggling on making this a viable idea on my own.

Basically, the theme of the campaign is the players all start at "the end", where they are all high-level characters in normal D&D 5e classes fighting the powerful BBEG. Just as it seems that they are winning, they are suddenly teleported by the BBEG into a strange new modern world, where magic does not exist (still deciding if I want a little magic at least for the campaign to work, though).

They will then all start with level 1 characters of classes that would mostly match up with their previous character's counterparts, and they would have to work and search in this new realm (kinda futuristic world) to find out how to re-open the portal and stop the BBEG.

In theory, sounds amazing. When I actually sit down to try to work it through though, it seems very convoluted and feels as if there will be a lot of problems making it work. If any of you have any suggestions on what I can do to make this idea come to life, I'd appreciate it! :)

Thanks in advance!

That seems like a hard thing to make work. You'd be building all-new classes, and D&D fits poorly when pulled out of its fairly narrow genre.

I suggest looking at other systems to see if any of them work for your group. (There are certainly some that can do both high fantasy and low/no magic modern, but the ones that immediately come to mind may not be to your taste.)

A variation that might be easier to do in D&D is pulled from one fantasy world to another, very different, one.

The other thing that comes to mind about this idea is that, while it sounds cool, starting right at the end means that the players have no emotional investment in the old world.

If I were to run this campaign, I'd use something like the Cypher system, not 5e, because it is much more balanced along magical/nonmagical lines.

You could replace all the spells and stuff with tech that does the same thing.

This is definitely a game I'd run using a different game system, like GURPS or Savage Worlds, rather than going with D&D.

One issue here is the idea that they all drop to level 1 for being transported. I'd struggle, if I were playing a barbarian, to agree that my high-level barbarian suddenly forgot how to swing because magic stopped working!

Transporting the party to a non-magical realm is definitely an idea. I have a similar arc planned for my world, where there is a place isolated from magic, and has been for hundreds of years. The magic is still there, locked in crystals, but they are controlled by a leader caste and only used for generating power for technology (imagine a wizard who comes to earth, knowing Lithium contains pure magic, to find us using it to power our cars and phones). If the spellcasters want to recover slots, they need these crystals. They will be seen as saviours or a menace, as nobody else can do magic (imagine said wizard can use batteries to cast spells). To make the casters not feel so dumped-on, the martials will find it hard to fight when the enemy all have laser weapons at their disposal. Once they start fighting, they can take the weapons and use them, or use the crystals for their spells. Thus, it rebalances.