For anyone having this issue, here is what happened in my case:
After a user enters a phone number and clicks Add the site sends a POST to https://public-ubiservices.ubi.com/v...honeActivation, in response it gets a JSON, and there's a very telling message inside, at least in my case: "The Phone Number cannot be associated. PREPAID"
You can check it for yourself, go to recovery phone setup, open DevTools (usually F12), navigate to Networking tab, then go back to the form, enter your phone number and click Add. You should see a request to startPhoneActivation in Networking, clicking that row would open a preview and in decent browsers, it will show JSON contents in a human-friendly way.
Check the "message" field yourself to find out exact issue you're facing.
Hey there everyone. If you are unable to add your phone number to your Ubisoft account, please check out our site here when adding your phone number. However, if you are still unable to add it then please review our site here.
If you are unable to add your phone number for 2-Step Verification, there is an alternative method by using an email for 2-Step Verification instead.
In regards to the request that Ubisoft Connect makes when you log in, to add a phone number, this is something that our internal teams are currently looking into. However, I apologize for the inconvenience as I do understand the aggravation that this can cause.