I would try standard ports from the lan side, i.e. 22 for telnet, 23 for SSH...
Also, for some TechniColors: SSH on port 6666 using root/root.
I don't think there is any standard for telnet ports on routers, depends on the manufacturer and their firmware version I suppose. Telnet has been insecure and obsolete for years, maybe they switched to SSH, or just have it turned off by default? Most would consider any router with open telnet port insecure these days, even if it is just on the LAN side.
You can run a portscan if the above ports don't work also.
Just my thoughts.
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