I saw multiple threads on forums about it, basically it isn't supported on the Three network (yet) until OnePlus put out a firmware update that enables the connectivity.
"Why is my 5G enabled handset not compatible with Three’s 5G network?
Handset manufacturers and all 5G Operators are eager to ensure that 5G mobile devices will get the best 5G experience. This means work is still underway with Three Ireland to support 5G access for a very limited number of 5G handsets. Visit three.ie for the latest list of 5G supported handsets (variants of which are all fully supported) if you are bringing your own 5G handset to Three"
So basically it's a waiting game for now. And you just need to keep an eye out for the firmware release.
