Accessing the server using the router's IP works inside the network, but not outside

So I have a domain (nextcloud.selfhosted.tamashalasi.com) which points to 91.101.56.134:

The public IP of my router is the mentioned IP, this screenshot is from https://whatismyipaddress.com/

...and this is from the router's page (note that although it is a dynamic IP address, the IP address only changes if the MAC address of the router changes):

I have a server on the network which has the name "debian". It has a permanent IP address of 192.168.0.101; the ports 80 and 443 get forwarded to this IP.

The server runs on a barebones debian 5.10.0, with nginx installed, running the default config with server_name nextcloud.selfhosted.tamashalasi.com added to it. I also added an extra rule to iptables, which I added to enable accessing port 80. This is the output of iptables -L (docker is also installed, but that shouldn't be a problem I suppose):

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http

Chain FORWARD (policy DROP)
target     prot opt source               destination
DOCKER-USER  all  --  anywhere             anywhere
DOCKER-ISOLATION-STAGE-1  all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
DOCKER     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
DOCKER     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
DOCKER     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Chain DOCKER (3 references)
target     prot opt source               destination
ACCEPT     tcp  --  anywhere             172.20.0.2           tcp dpt:http

Chain DOCKER-ISOLATION-STAGE-1 (1 references)
target     prot opt source               destination
DOCKER-ISOLATION-STAGE-2  all  --  anywhere             anywhere
DOCKER-ISOLATION-STAGE-2  all  --  anywhere             anywhere
DOCKER-ISOLATION-STAGE-2  all  --  anywhere             anywhere
RETURN     all  --  anywhere             anywhere

Chain DOCKER-ISOLATION-STAGE-2 (3 references)
target     prot opt source               destination
DROP       all  --  anywhere             anywhere
DROP       all  --  anywhere             anywhere
DROP       all  --  anywhere             anywhere
RETURN     all  --  anywhere             anywhere

Chain DOCKER-USER (1 references)
target     prot opt source               destination
RETURN     all  --  anywhere             anywhere

With this config, I can access http://nextcloud.selfhosted.tamashalasi.com/ inside my local network, but can't outside it. Accessing the IP directly yields the same results.

I would like to make the IP accessible outside my local network; do you know what might have gone wrong?

_{Note that I am a total server setup noob. If this post doesn't belong here, I would love if you could point me somewhere where the post does belong so that this issue can be solved. Thank you ❤️}