Try changing your NAT rule to
“iptables -t nat -A POSTROUTING -s 10.11.13.0/24 -o eth1 -j MASQUERADE”
Also try changing your AllowedIPs to 10.11.13.0/24
The other issue may be the ability of the host/server to reach the IP address of the pihole macvlan interface. Have a look at this page, it’s helped me out before -> https://adventurenotvacation.com/2020/12/01/docker-macvlan-and-debian/
Data caps on home internet services should be illegal. They should also be much higher on mobile, but that’s a whole other topic.
I have 940/940 Unlimited FTTH for $93.45(Canadian).