Make sure you are using they new SIM that supports 5G and their new network. If you are not sure if you have their new SIM, then just go to the store and get one.
Try shutting down the phone and remove the Sim card and wait a few minutes then put Sim card back in maybe the Sim card is damaged or not sitting in cradle correctly
I'm currently on t-mobile and so far no issues on my fold or flip. I also live in a middle size city, that could be why. Not sure if you have it enabled but disable 5g. It saves battery and I have a better connection. It's the first thing I do no matter the device. LTE still seems to work better at the moment.
Odd. Yeah I've owned both of these. The Note was unlocked from Samsung and the iPhone was Tmobile branded. Same issue. Once I switch off the 5g on both I was good to go. Maybe we will see an update soon the resolve the issue one can hope. I would disable it though just to check as a troubleshooting measure