A poor signal can put a strain on batteries but I haven't seen a device overheating as a result unless there are other apps causing the issue. Can you post your battery stats? Does the phone tell you it's overheating or is it hot when you touch it?
Some say if the phone feels hot it's because the heat is dissipating. There's an app on Play store called HW Info. Can you install and check out the battery temperature when it gets hot and report back with what temperature it shows. In the meantime, clear system cache and if you can upload a screenshot of your battery stats, we can check for any rogue apps.
I have definitely noticed that persistent attempts to use mobile broadband in poor/S21 poor signal areas creates overheating and kills the battery. Yet again, something else with the S21 that Samsung ****ed up.
Recently I have noticed my phone heats up with 5g even though its a good signal. But I don't use phone much outside so will see how it goes. I specifically ordered a 5g sim so don't feel like going back to 4g. Also in the winter it would be nice to have something warm down there .😁