Hi. There is an old topic about custom domain. The recents updates from this topic says that:
We are working on subdomains per app right now (see here in our roadmap app), which will be a precursor for completely custom domains!
So, for this moment, there is not much options that don’t use iframe. For the main question, I don’t think there is much options, it seems to be a client side configuration/network that blocks iframe in both cases.