Is Signal (app) blocked on Mobile Data?

It shouldn’t unless your mobile carrier is detecting and blocking it for some reason.

But, use Session app instead. No email/phone for signup, and your messages are onion routed through community nodes. Check it out if you haven’t already at getsession.org

Alternatively, use a Matrix client with a throwaway email account from Tutanota.

At least in the US, I've never experienced this nor have I heard of any others that have.