Phone not accepting Sim Card
So my old LG G4 (H815 Factory Unlocked) isn't playing ball with my Boyfriends 3 Sim. The phone is confirmed Unlocked and I've been able to use my GiffGaff sim in the phone which further confirms it's unlocked. However it refuses to accept a 3 sim (Both my new 3 Sim; his and one from the store)
I've tried factory resetting the phone a number of times, rebooting, opening it up and inspecting the prongs. Nothing; any advice?
I've also fiddled with APN settings and LTE/GSM settings.