Very poor signal reception

Hi there - I just migrated from GiffGaff to Smarty but the signal I get is incredibly low, just one bar! I live in W3 and based on the coverage map, I should get a nice 4G signal indoors and outdoors. I reset my network settings, also switched from SIM to eSIM. I have a much older phone with a GiffGaff SIM card and definitely gets more signal. Any ideas / suggestions? Thanks.