SIM card inserted perfectly, but no signal

At first, I thought that my patchy signal (no 4G, have to retry sending texts 3 times before they actually send, no calls work) was because of my provider (Tesco), but I asked them what it was. When they said to swap the SIM with another phone, my SIM worked perfectly, but the signal on my actual phone did not. I've restarted and reinserted my SIM countless times. I have literally no idea how to fix this. There isn't water in the compartment, either. Any ideas?