Apple Watch / Error Code 88

[rg11]

I'm awaiting a callback from an engineer, but figured I'd post here in case anyone can offer any insight into what's happening, or tell me of something I can do to speed things up. 

Signed up with O2 a few days back, to get cellular service for my Apple Watch S3. My phone is an iPhone SE. 

Phone and watch are paired fine. When I try and set up mobile data I get asked to sign into O2, do so, and then get told something went wrong, but to phone 202 and quote reference code 88. Did that yesterday and they took my EID (is that the esim?) from the watch and promised a callback. 

Possible problem is that I was on the iOS beta program. I've left now, but am still running 13.3 (current is 13.2). I could downgrade, but am reluctant to unless I know that IS the problem, rather than COULD BE. 

Other possible issues - new account, older iPhone and older Watch, neither purchased directly from O2. 

Any ideas what might be happening?

[MI5]

@rg11 

We know the series 5 watch doesn't work with beta software, so that's your most likely issue.

The fact that you didn't buy from O2 is irrelevant.

[rg11]

Restored back to iOS 13.2.2, the latest official release, still getting the same message and no callback. Will try 202 again, I guess.

[MI5]

Do you have the correct carrier profile installed and did you reset watch and run setup again?

[rg11]

Restored phone to 13.2.2, reset phone and set up from scratch. Carrier settings look fine,but can check something specific if you have any ideas.

[rg11]

Carrier version is 39.0

[MI5]

In theory, it should all be working now but it can take 48 hours to activate.

[rg11]

Been told to wait up to five days for a callback. Only moved to O2 to use the Apple Watch, if it doesn’t get fixed I’ll probably leave within cooling off period.

Can’t see what needs to “activate”. If I got a message saying “All good, wait 48 hours” that’d be fine. Will hope I actually get a callback.

[MI5]

It's O2's old slow systems that are the problem.
Even simple tariff changes take 24 hours.

[rg11]

I don't think that's the issue, there's a technical problem somewhere, rather than needing to wait.

I got a message from an engineer asking me to unpair / remove from iCloud all watches, then try again with the watch I want to use, which I did. Exact same problem still happening though. Took me about 30 minutes on hold to get a promise of another callback. 

So if anyone's got any ideas - Iphone SE, iOS 13.2.2, O2 39.0, Apple Watch S3 6.1, no other watches paired to phone or showing up on iCloud - I'd be very grateful.