Claims "No SIM restrictions" - but shows "SIM not allowed"

So I've been given my brothers old iPhone as I needed a phone after my old one broke, and he has had no issues with it in the past. I've factory reset the iPhone and everything and set it up from there as usual, however when I put in my old SIM card into the phone, in the general settings under the "Physical SIM" section, it says "Network: Not Available" and when I click on it, it states "SIM not allowed".

However I doubt that it is locked as on the section just above the "Physical SIM", there is a line that says "Network Provider Lock: No SIM restrictions". So I am confused on what to do. I'm with GiffGaff as my mobile network provider, as was my brother.

As mentioned as well, I have completely factory reset the phone so all the settings and stuff should be set back to default. What can I do to fix this issue?