They are not usually open to talking about KYC issues with third parties like SBR but we can give it a try if you want to send in a
Sportsbook Complaint form.
Your best bet is to offer any other form of ID that might help them approve you. And if there is any evidence of you being called by this nickname over time. Like over a series of facebook posts for example. Maybe send them that.
If you can't somehow negotiate this directly with them then the only other real option will be an appeal to the MGA, their regulator, who will most likely just uphold what the book says in a thing like this. :\