Try deleting your internet settings and reconfiguring them from scratch using these instructions:
If it is still bad then try switching off, taking the SIM out, gently cleaning it, reinserting it, switching on, manually scanning for networks and connecting to 48.
If it's still bad then you may need to try a replacement SIM.
And you may want to contact 48 support for more advice.
See the link in my signature below.
Hope this helps.