Unable to reach self domain name from inside LAN network

It seems that this question is very similar to I suddenly can’t access my own web server within my LAN via the external WAN IP address but nothing there helped me so I'm trying again.

I have a router with WAN address 1.2.3.4 and local network 192.168.1.1/24. I have a web server running on 192.168.1.100.

In router settings, I have changed the port of management panel from 80 to 81 not to interefere with the webserver. Then I have added a port forwarding rule for TCP 80 -> 192.168.1.100:80. Finally, I have assigned a domain name example.com to 1.2.3.4 so accessing example.com correctly routes to my web server. From the world.

However, when I try to enter the example.com from any of the devices connected to the 192.168.1.1/24, it does not work.

I tried to diagnose it with ping, and the domain correctly resolves to the IP address so it seems like not a DNS problem:

root@linux:~# ping example.com
PING example.com (1.2.3.4) 56(84) bytes of data.
64 bytes from 1.2.3.4.ip4.feromedia.eu (1.2.3.4): icmp_seq=1 ttl=63

Also, traceroute stops at the router:

root@linux:~# traceroute example.com
traceroute to example.com (1.2.3.4), 30 hops max, 60 byte packets
1 192.168.1.1 (192.168.1.1) 0.350 ms 0.256 ms 0.195 ms
2 * * *
3 * * *
4 * * *

I have found neither NAT Loopback nor NAT reflection settings in my router as suggested in the question linked above. I have not configured anything in the static routing and I can't remember anything I could done that would cause such behavior.

My router is DrayTek Vigor2925.