I don't know exactly how your watch behave, but I had a similar issue on watchOS 8 a few months ago (not on the beta), I couldn't add any plan to my watch. My mobile data provider had to reset something in their system so I can connect a plan again. I am currently on watchOS 9 at the moment, and not having issues with cellular connection, although I didn't add a plan since installing the beta, as it was already enabled. So I'm not saying it can't be related to the beta, but maybe look with the support of your data provider to see if it's something up to them