IPhone 12 Big Issues

Upgraded from iPhone xr to iPhone 12 same network contract phone. Business contract so unlimited data no restrictions on hotspot etc. Yet constant problems with connections, really weak signal, slow responding won't open many apps, when it does it will soon crash or go off so far it's the worse phone l've ever had.

I've done all the tips and tricks with turning it on and off refresh settings and rebooted.

Is the any thing else anyone knows of to try to fix the problem or do you think it could be the general faulty phone?