@trivial_factor 

Hi,

The settings in the menu "Advanced" -> "Network" -> "Internet" relate to the network that is connected to the router's WAN port.

If you have the Pi-Hole connected to your router's LAN network, then all you can do is to enter it's IP address in the "DNS" field(s) under "Advanced" -> "Network" -> "DHCP Server".
Client devices on the router's local network that obtain their IP configuration via DHCP will then be assigned that DNS server. 
 

  @trivial_factor Thank you. Does this mean I have to enable DHCP server in Pihole and reassigned all static IPs (already done so in the C80)?

  @trivial_factor OK, got that working. Thanks a lot for your support.

  @trivial_factor Well it's more complicated than that. The reason I installed Pi-Hole in the first place was to try and understand why my devices were not resolving my domain name when connected inside the LAN. They *do* connect properly if I access the network through a hotspot.

Some tests I did:

Device inside LAN (192.168.1.5 is my server running Pi-Hole):

> cat /etc/resolv.conf

nameserver 192.168.1.5
nameserver 192.168.1.1

> scutil --dns

DNS configuration

resolver #1
  nameserver[0] : 192.168.1.5
  nameserver[1] : 192.168.1.1
  if_index : 11 (en0)
  flags    : Request A records
  reach    : 0x00020002 (Reachable,Directly Reachable Address)

> nslookup subdomain.mydomain
Server:		192.168.1.5
Address:	192.168.1.5#53

ping mydomain -> OK
nc -zv 192.168.1.5 22 -> succeeds
nc -zv mydomain 22 -> *** fails *** (as do all other requests, 80, 443...) ***
dig mydomain +short -> 192.168.1.5

From outside the LAN (hotspot):

> cat /etc/resolv.conf
nameserver fe80::cd:feff:fe6b:5c64%en0
nameserver 172.20.10.1

> scutil --dns            
DNS configuration

resolver #1
  nameserver[0] : fe80::cd:feff:fe6b:5c64%en0
  nameserver[1] : 172.20.10.1
  if_index : 11 (en0)
  flags    : Request A records, Request AAAA records
  reach    : 0x00020002 (Reachable,Directly Reachable Address)

> nslookup subdomain.mydomain
Server:		fe80::cd:feff:fe6b:5c64%11
Address:	fe80::cd:feff:fe6b:5c64%11#53

Non-authoritative answer:
subdomain.mydomain	canonical name = mydomain.
Name:	mydomain
Address: my public ip address

ping mydomain -> OK
nc -zv 192.168.1.5 22 -> fails (as expected)
nc -zv mydomain 22 -> succeeds
dig mydomain +short -> my.public.ip.address (as expected)

The only answer I get from ChatGPT is that my router doesn't support NAT loopback and that I should either edit /etc/hosts or add local DNS entries in Pi-Hole. But:

1. I can't modify /etc/hosts on all connected devices.
2. Adding local DNS entries in Pi-Hole doesn't seem to work.

To be clear : this problem is not related to Pi-Hole. I was having the same issues before installing Pi-Hole. I only installed Pi-Hole in order to try and resolve this issues, by having a local DNS server.

Is the C80 incompatible with "NAT loopback"? And how can I resolve this?

Please help. Thanks. 

EDIT: ssh to public ip

Although the router forwards port 22 to the server, ssh to my public ip doesn't work either. It does work if I ssh to mydomain from a hotspot (outside the lan), or if I ssh to 192.168.1.5 from within the lan. What is causing this?