I normally don’t use the mostr.pub relay and I have been able to see posts from activity pub accounts. I connected that relay recently to speed up load times in Nostr.
Discussion
You can also use the Nooti app to cross post on both. My sense is that you get more traction on Nostr with a Nostr account.
Why do you think that is nostr:npub1j60x528w2g2vkq5kae5uhh8y7sezjyj20zcsg0v9muc72cmdpu0s0md7ua ?