Did this coincide with the switch to 5g? The way it evolved for me was:
Had 4g iphone 6 or 7, and amazing service/cell data through 2019
Service started getting spotty throughout 2020 but I thought it might have been my old ass (company) phone
Got a personal 5g iphone 12 december 2020, legit fucking sucked anytime the 5g icon was on. Significantly slower than when it showed 4g/LTE, and also significantly slower than the 4g/LTE on my other old ass phone.
Sometime in early 2021 I stopped seeing the 4g/lte icon and the service became universal shit, and it's only declined since.