Its very easy to solve this problem, i was also facing the issue but currently i have solved it.
Reset your network settings and your problem will be solved
[link|https://okadtech.com/samsung-s21-signal-problem-solution/|https://okadtech.com/samsung-s21-signal-problem-solution/|new_window=true]
You can refer to this article, this will help you in resetting your network setting.
Go and solve your problem and thanks me later.