Carriers have an optimising function which used to be called voting (no idea if is this way) where the connection between a phone and a tower is determined by geographical mapping of signal strength. Think: a tech driving around neighbourhoods with a briefcase computer logging signal strength from each of their towers.

That data governs which tower subscriber devices use. If this optimisation is insufficient, or if circumstances change which affect signal strength, line of sight, interference etc. your device may be connecting to a more distant tower.

There are apps that can display a map of all radio towers, the carriers that have transceivers on those towers, and which tower you are currently connected to, along with signal direction and strength data. Most phones also have a diagnostic mode you can enable which provides more data about your device’s cellular connection.

The data from such apps can be useful if you raise a complaint with your provider - of course no guarantees they’ll do anything, but providing them with quantitative data to support your request gives you the best chance of their taking action.