I think it's quite late for Roobet if they want to hire a brand ambassador for the world cup promotion since it's already 3 days before the first match will happen. Maybe Roobet only looking for UFC and boxing first since their mascot is kangaroo which is usually linked to martial art or boxing. It wouldn't be easy too for casino to hire a football player to become brand ambassador, I've remember there's a problem to the player when he promote gambling site, but I can't find the thread.
What casinos often do in football is to become a club sponsor, not a player sponsor, some bookmakers have done this either as a sponsor for a club or a national team, but will their bookmakers logo appear on the team's national costume at this World Cup? is still a question because of the considering of the host country
I think there are no sponsors for the national teams as far as I know, thats not allowed in any nation from what I can see. The jerseys that nations have has absolutely no ads at all.
This means that it would be just a person if you want to get promoted, in that case the person you hire as ambassador could make instagram stories and all that about your casino and there wouldn't be a problem about it, not in Qatar or not in anywhere, because you would just do it on your hotel room and would shoot a story and would promote it and thats it, hell change your instagram bio if you want and add roobet to it, who could say anything about that? Would be a lot more possible and easy.