I set up my oc9. Everything runs smoothly I have friends that access the server with their users and use it as intended.
Yet my parents have trouble accessing it: If they can connect it takes forever. Yet, mostly Browser times out. When I use my mobile in my parents WLAN I have the same issue when I sit at the same place using 4G I everything works as it is supposed to. The bandwith of my parents network is not the problem (Youtube streams in 4K). Hence it's got something to do with the settings - most likely in the router. Yet, I have no idea what I should look for. Any ideas?
DNS issue, or a wrong/incomplete ipv6 config (of your server or your parents network).
To test ipv6 is very easy, open about:config in Firefox, set the variable network.dns.disableIPv6 to true. If that is the issue, you can enable ipv6 again and test your parents network with: http://test-ipv6.com/.
You can use traceroute to check the network path. There it should be obvious if the routing table is wrong. Firewalls could also block your connection. To open the page, you can also use curl on command line (Linux) and see the whole connection process and this way, you can perhaps see what slows it down.