It is not an issue with my SIM card. I think it was fine with ios 18 beta and also with my android device. The connection loss happens every few minutes. It loses signal gradually until it is totally disconnected. Then, all of a sudden, it gets a full signal. The situation I mentioned occurs within 10-60 seconds. I live in Seoul and I am not in a basement or somewhere that causes connection loss. It is totally a software issue.

Exactly the same issue.