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It's a guide only but it does give an idea what's happening in your area. What sort of phone is it and how long have you had it? I assume the signal strength on the phone is down to one or no bars at all when inside?
Try it on 3G/ 2G auto connect and see if it will work inside. Personally I think there is something wrong with your phone or the sim as my S7 works faultlessly on 1 or 2 bars and you have 3.
Short of testing the sim in another phone to see if it works inside I think if it was my phone I would be doing a full factory reset. You haven't dropped or damaged the phone in any way?