You might have to debug what causes it using a cell connection monitor app.
Cell towers may kick off phones with a weak signal when congestion is high. You may also get kicked off if your phone is using invalid channel aggregations because it was made for a different region (or is programmed wrong). For example, T-Mobile will kick off an NSA 5G phone if it does nonsense like mixing a short-range LTE band with a long-range 5G band. That combo requires SA 5G.
