I have the same issue. Here is a quote from my wife this morning;
"My iPhone 7 has all four bars.
iPhone 12 Pro Max is sitting right next to it and his two bars."
I had been a very loyal business customer of Sprint for 12 years. I have spent the last 3 solid days talking to 3 different countries and I can't tell you how may people trying to activate my iPhone 12 pro Max. After being told by 4 tech reps the SIM card from T-mobile, that came with our 2 brand new phones, was wrong and we needed a Sprint SIM card which they then ordered and sent out. Went out regular mail and took 4 days. While waiting after 3 days, I spoke to another senior tech at T-Mobile in the Philippines that was able to activate phone with the original SIM card from T-Mobile.
Day four... Took a drive in $65,000 car with Car Play and was unable to connect. Called T-Mobile Tech-PHD line and phone dropped the call 4 times trying to get help. After 5 calls to five different people the last one referred me to Apple and said it appeared to be an issue with the Apple phone or software. Apple tried numerous times with me and had me reset phone which was completed last night at 12:30. Had to re-enter passwords, previously set up, next morning. Apple scheduled a call at 8:00 am and someone from Apple called at 8:06 on my Apple cell#. I gave her my land line and she said she would call me right back. It is now 8:25 and still no call back.
To sum things up I gave T-Mobile a 10/10 on ease of ordering the new iPhone 12 Pro Max. Service from T-Moble trying to get phone activated and addressing technical issues I would rate a 1/10. VERY POOR SERVICE
It is now 8:30 and still no call back from Apple.