Help 4G issues

[gadusmorhua]

I have a Samsung Galaxy J5, specifically the SM-J500G model. I have two SIM cards fitted, Tesco O2, and GiffGaff, which also uses the O2 network. Tesco sent me the APN configurations to enable 4G, and comparing these to my mother's, on her J5 handset, they seem correct. I have also selected LTE auto select in the menu. The GiffGaff settings I had to enter manually, but these seem correct as it allows me to connect to the internet, speeds up to H+, the same being true of the Tesco SIM. My mother gets 4G constantly, but I cannot, even when sat next to her. As the issue involves both SIMS I'm assuming it is something on the handset, but what? I have updated the software, rebooted etc. etc.
Help!!

Thanks,

Gads