I would separate the burger from the bun at the restaurant, in the go box. If the bun is already soggy, just leave it.
To eat leftover burger:
Make a pasta side of macaroni and cheese, fettuccine Alfredo, or similar. Set aside.
If you’ve brought both burger and bun, reheat the burger on low under a minute, and then the bread, no more than 10 seconds.
If it suits you, reassemble and eat with your side.
If the burger is decent, but the bun is sogged, toss the bun, and either eat the burger with a fork, or chunk up and stir into the pasta. Eat.
If the burger sucks and the bun is soggy, pass them up. Fry yourself up a nice piece of garlic bread, pour a glass of wine, and eat the pasta.