karna46 even in a 5g phone it goes to 2g network
giffgaff does not offer 5G yet I believe.
Your screenshot shows H+, which is the fastest form of 3G available.
What rough download speeds are you currently achieving in Mbps?
karna46 service issues have been reported in my area yorkshire.
These issues may still be ongoing, possibly due to the weather. Have you compared to other local O2 / giffgaff users around you if they're experiencing similar.
Was the mobile data previously performing significant better in the same location? If so, you may have to sit tight until things are fixed. It's up to O2 to maintain the masts.