Setting up a mobile data plan for an Apple Watch

[S2PZ]

I have an Iphone 16 with O2 sim only plan 40gb.

I have an Apple Watch 9 GPS and Cellular which I did not purchase from O2.

I have tried to set up a mobile data plan for my watch, but get error code 90.

I do not need a separate number, I want all calls to come to both devices (ie I don't want 2 numbers), and I want to be able to use/share my 40gb data allowance on phone SIM plan on the watch.  

O2 support say they can ONLY sell me a mobile data plan for the Apple Watch with 1GB of data (not enough) and that the data from the phone plan cannot be shared with the watch (I only use 8gb or so average per month).

They also say I cannot buy another SIM only plan to use with the watch and that I have to have specific watch plan.

Everyone at O2 I speak to tells me different info and I am totally lost.  Can anyone shed any light on how to get an airtime plan on my watch for as little ££ as possible?

many thanks

[MI5]

@S2PZ 

It's £7.00 a month or 7.75 with the yearly increase and you need the watch plan.

Data is shared when phone is in range and only uses watch data when remote.

Open the watch app on your phone and follow the prompts to add a data plan.

Issues covered here Activate Apple Watch Guide 

https://www.o2.co.uk/help/phones-and-devices/smartwatches/apple-watch

[S2PZ]

Thanks for the reply.

Is it correct that the watch plan only has 1GB data allowance?

I have followed the prompts and get error code 90

Will follow the instructions in the guide you have linked to.

[MI5]

@S2PZ 

Yes 1gb on the watch plan has proven to be sufficient for normal expected usage.

Also, lots on here regarding error 90 https://community.o2.co.uk/t5/forums/searchpage/tab/message?q=error%20code%2090&sort_by=-topicPostDa...