Sometimes when you get issues with one device working and not another, it can be to do with IPv4 / IPv4. Since modern devices will generally happily resolve IPv6 but older ones won’t. Do you have both a A record and a AAAA record set up for your custom domain?
If so, what error message do you see when you try your own domain?
Does it work when using curl? That can reveal a bit more about headers and whatnot. If you have curl, if you open a terminal window and try e.g curl -I https://your-site.com … what does that reveal? Should be a load of headers, hopefully returned from Fly! If not, the headers could show where the request actually is going.