If your router has a public IP address, you can still validate that IP address.
HTTP/HTTPS validation is the only method available for IP address validation. The HTTP/HTTPS validation method consists of adding a TXT file on your IP address and having Sectigo scan that IP address and validate it. There is no way to upload a TXT file on your router. The solution to get the IP address validated is to reroute the IP address to a server, put the TXT file on that server, pass the IP validation, and then reroute the IP addresses back to the router.
You can read more information on what the TXT file should include and where to upload it in the following FAQ item: https://www.ssldragon.com/contacts/faq/#collapse-14363