I found a solution but I am unsure if this will break anything else:
This is what I did:
sudo nano /etc/systemd/resolved.conf
[Resolve]
DNS=192.168.88.1 #uncommented & my routerip
#FallbackDNS=
Domains=lan #uncommented & lan
#LLMNR=no
#MulticastDNS=no
#DNSSEC=no
#DNSOverTLS=no
Cache=no #uncommented
DNSStubListener=no #uncommented
#ReadEtcHosts=yes
sudo systemctl restart systemd-resolved
Now I see port 53 is available and I can install Pihole via Docker!
BUT it seems this messes with /etc/resolv.conf as well. By default:
GNU nano 4.3
# Generated by NetworkManager
nameserver 127.0.0.53
But now it contains (note I didn't do this, this happened by itself)
nameserver 192.168.88.1
nameserver 37.143.84.228
nameserver 37.143.84.229
search lan
That first nameserver = my router IP
the 2nd and 3rd are the 2nd and 3rd DNS providers (fallback) shown in my router.
Is this a safe solution?