Hi @trubulmaker, did you ever get a resolution on this issue?
I have what sounds to be the exact same problem. I was coming off of an iPhone 8 with T-Mobile which worked great and had no issues other than battery life at this point. Switched to the new iPhone 13 Pro at the end of 2021 and have had nothing but problems since. If I'm not on WiFi, I cannot use my phone. Texts go through occasionally (when sent at Text Message, not iMessage), calls occasionally go through, and VERY rarely can I use the internet.
I've been in store with both T-Mobile and Apple, been on tech calls with Apple, switched SIM cards, got a 2nd new phone, and nothing has changed. Apple blames T-Mobile, and T-Mobile blames Apple.
It makes no sense. I'm in the same home as previously, verified that all my settings are fine, and my wife also has the 13 and T-Mobile with no issues.
I will be dropping 1 or both of these companies very soon if I can't find a solution. I've wasted way too much time on this, and you cannot function as a traveling working professional currently without being able to use cellular data.