Firstly I would suggest that you check whether you are connected to the right mobile network. To do this, find and tap Settings > Network & Internet > Service providers > Choose the network of your network operator.
Secondly, have you made sure that you have set your device to use the network type/mode your SIM card supports? if you do not know which network types are supported, I suggest that you contact your network operator. To select a specific network mode, follow the instructions below.
Find and tapSettings>Network & Internet>Mobile network.
Select SIM card, if relevant.
TapPreferred network type.
Select a network type.
If none of these measures manages to improve the signal quality, It would be great if you could try another SIM card on your phone. If the signal strength is better with another SIM card, your SIM card may be damaged and in need of replacement. In that case I suggest that you get in contact with your service operator.
Try these measures and feel free to keep me posted on how it goes!