Realme C21 Network Problems

The Realme c21y is a brand-new phone that I just bought, but it's giving me some trouble. The device doesn't seem to be able to get a 4.5G signal in places where other phones don't have any trouble. I've tried SIM cards from different service providers, but the problem hasn't gone away. Changes to the phone's screen settings don't seem to help either. I'm very upset that this problem has come up only a month after I bought the device. I've even gotten calls, but the phone looks like it's off and the callers can't get in touch with me. I think the problem is with the phone's network because I've had trouble with the signal from the start. I'm asking for help from the community to solve this problem as soon as possible.