"Antennas then and now: Have a 2017se that has a weak cellular signals in some locations Have a 2020se that has a better signal there (still not great though) Is this due to the antennas in the newer phone? OR perhaps the exact BANDs in the phones?"
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iPhone Inconsitencies:
I take it you are living in a rural area. Both iPhone work off of up to 5G. If there is no 5G, then it's only going to work with what is available to you. So, I'd says its a hardware failure, given it is the same provider, used in the same location. As more-and-more people move about, Cellular services will of their part in making service available.