Which hub/router are you using, as AFAIK, the PN Hubs do not have that option? A screenshot of the display would help.
@Brigadier welcome to the forum.
Plusnet Hubs do not offer a guest wireless option, which hub are you using?
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Assuming that it's a Plusnet Hub 2 try switching it off, leave it for 30 seconds, then back on.
If that doesn't work then factory reset it by pushing a paperclip or similar into the reset hole. Keep the switch closed until you get a green light then leave it to reconfigure itself.
These hubs use adapted BT Firmware and it sounds as though yours has booted incorrectly
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