Do you mean with “custom dns” a solution like this?
No, haven’t tried yet. Btw, in the meantime I learned that it seems not to be possible to point a FQDN (like mydomain.dedyn.io) to an internal IP address with the FRITZ!Box 7490. And it doesn’t support editing the DNS like it is described here: https://yunohost.org/#/dns_local_network_en