Home page showing PPPoE DNS, while you changed is for SLAAC/DHCPv6 and DHCPv4 DNS.

Make sure you use correct ID.

Enter User Name: tmadmin and Password: Adm@xxxx (xxxx is the last 4 digits of MAC address)

it seems the new fiberhome only have tmadmin user, no superadmin user

can try fiberhome and password fiberhome

Just wonder, did ur DHCPv6/SLAAC or DHCPv4 DNS don't get changed?
Unless you wanna setup NAS/ftp etc via USB on your own router's firmware, it should not affected too much?

I also prefer letting device connect to the DNS directly, so it allow to enabled secure DNS(TLS or https whatever)instead of router's DNS.
Unless you also want DNS caching by router?

And to change default DNS of your PPPoE, it might also be on the PPPoE setup, try check that one.

After you applied your screenshot, check your phone's wifi and your PC's IP address and DNS, it should be the one you setup.

So it will be bypassing your router DNS relay, and connect directly to the DNS server.

Also remember, setup for IPv6 as well, because, by default all device will try IPv6 DNS to obtain both IPv6 and IPv4 address, only if IPv6 is down, then it will be IPv4 DNS.