Thought I'd jump on this thread as I'm having the same problems as OP, let me know if I need to start a new one. I have no options for 5G in my network settings. I have tried turning the phone off, removing the sim and restarting to no avail.
I'm using an unlocked OnePlus 8 with a pay monthly SIM. I've had 5g coverage before with a previous network, but nothing since joining Virgin.
I meant to reply to this sooner, but I'm sorry to say that a new SIM did not fix my issue 😞
My phone still shows the same options as before, so I'm out of ideas. I've been recommended to try a factory reset, but that's a bit inconvenient at the moment so I think I'll just end up leaving it for now