Ping the uplink device via NAT

I have two routers. One is connected to the Internet, has a WAN IP XXX.XXX.XXX.XXX, internal IP 192.168.10.1, and DHCP server 192.168.10.* The second router connects to the first one via WAN and gets a DHCP IP 192.168.10.213 with Gateway 192.168.10.1 (the first router). It also has NAT with its internal IP 192.168.8.1 and DHCP 192.168.8.* If I connect via ethernet to router 1, I can connect to 192.168.10.1, but if I connect to the router 2 and try to connect to the router 1 (192.168.10.1), I get the following error: 92 bytes from console.gl-inet.com (192.168.8.1): Destination Port Unreachable

Question by IvanD on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.