Must be a dns thing or routing thing. Describe more where is the server located relative to your phone, how is the network setup...etc
Discussion
I am connecting to a Linksys Smart WiFi router with DNS setup to use my private PiHole instance. DNS works in all other apps except the Nextcloud client and other users have had problems with IPv6 bring enabled on the router. I've disabled it on both my mesh router and my modem / router but that doesn't seem to have fixed it. I don't understand how I can have an IPv6 IP address when it is disabled at the router.
