I made you the owner of the original guest post to make it easier for you to find.
Do you know which towers the phone is currently connecting to? It sounds like what might be happening is that the phone is dropping down to 2G or 3G. 2G and 3G either have already sunset or will sunset soon for all major US carriers. (2G is somehow still active on T-Mobile, set to shut down in Jan 2023, so maybe your current SIM is using T-Mobile towers.) Read more here:
https://www.phonearena.com/news/t-mo...-date_id134159
The question of why your phone is dropping down to 2G/3G may be due to poor 4G/5G reception. 5G can be notoriously fickle, depending on the wavelength, with poorer range and ability to penetrate walls than 4G, so if you can, try changing your Preferred Network Type to 4G (i.e., turning off 5G).