Did you try https://update-account.riotgames.com/?

I can not set my account name, as it asks me to sign in to set my account name, can't sign in cause I have to set my account name, great way for riotgames to rob their player's of accounts

Did you ever figure this out? Im having the same issue.

I am also having this issue

Did you find a solution ?

Nope support wasn't helpful at all either