Reset Circle admin passcode?

Hi all, I have a Netgear Orbi mesh system (RBR50 / RBS50) which has Circle enabled. However, when I try to administer this from the iOS app, I'm asked for a 4-digit passcode which I don't have. I do get the option to SMS the code to me, but that doesn't appear to work. How can I get this passcode reset? 

I had a trawl through the forum before posting and, three years ago, ChristianR appeared to be the gateway to getting this resolved, but I don't know if he's still doing this! 

Any help appreciated, thanks, Dave