I also have a TG862G, but from Xfinity (a Comcast company).
The default admin page is @ http://192.168.100.1/ & http://10.0.0.1/, but neither of them have a /cgi-bin/tech_support_cgi.
However, I discovered this page: http://10.0.0.1/wireless_network_configuration-1.php (probably also exists on 192.168.100.1) which looks like a secret wifi config page that has more advanced options than the normal wifi config page @ http://10.0.0.1/wireless_network_configuration.php (I found the wireless_network_configuration-1.php file by viewing the source of a few pages on 10.0.0.1, it was hiding in some HTML comments).
On the normal wifi config page, you can only edit the settings for your "Private" wifi hotspot, but on this -1.php page you can also edit the two "Public" hotspots: "xfinitywifi", and one that (on mine) looked like: "XHS-A6B18523". Since you can edit these two "Public" ones, you can also viewed the stored WPA key for XHS ("xfinitywifi" has no key).
Once I grabbed the XHS-* key and connected to it, I received a 172.16.12.100 IP (a subnet I've never seen on the other access point). On this one the gateway IP was 172.16.12.1. Nmap shows these ports on that gw IP:
443 = NET-DK 1.0 (ssl)
5001 = Arris/1.0 UPnP/1.0 miniupnpd/1.0 (Status: 501 Not Implemented)
8080 = (SIP end point; Status: 501 Not Implemented)
and same as the above for ports 8081 & 8888 & 5540.
All of those SIP ports were just HTTP servers that looked exactly like the customer version you see on http://10.0.0.1/ , except that my admin pass didn't work on it (tried the defaults too, plus some guesses).
When I went to https://172.16.12.1/ it redirected me to /cgi-bin/status_cgi, which contains a link to /cgi-bin/tech_support (which redirects to /cgi-bin/adv_pwd_cgi).
So maybe you could try all of that to see if your TG862G works the same :-)
P.S.
I tried the password of the day thing but the seed must be different on this one, and the SNMP thing doesn't exist on any of these webservers.
Thank you for reply, but I don't have xfinity firmware anymore, cause TWC wiped it out and wrote their own. It seems I don't have wireless_network_configuration.php pages anywhere and it doesn't broadcast wi-fi at all (I actually can't enable it, there is bug or something, it just shuts down), so I can't try that subnet either.
This is frustrating, cause I do have a password to /cgi-bin/adv_pwd_cgi but I can't find the SSH/Telnet options to enable.
However, I discovered this page: http://10.0.0.1/wireless_network_configuration-1.php (probably also exists on 192.168.100.1) which looks like a secret wifi config page that has more advanced options than the normal wifi config page @ http://10.0.0.1/wireless_network_configuration.php (I found the wireless_network_configuration-1.php file by viewing the source of a few pages on 10.0.0.1, it was hiding in some HTML comments).
On the normal wifi config page, you can only edit the settings for your "Private" wifi hotspot, but on this -1.php page you can also edit the two "Public" hotspots: "xfinitywifi", and one that (on mine) looked like: "XHS-A6B18523". Since you can edit these two "Public" ones, you can also viewed the stored WPA key for XHS ("xfinitywifi" has no key).
Once I grabbed the XHS-* key and connected to it, I received a 172.16.12.100 IP (a subnet I've never seen on the other access point). On this one the gateway IP was 172.16.12.1. Nmap shows these ports on that gw IP: 443 = NET-DK 1.0 (ssl) 5001 = Arris/1.0 UPnP/1.0 miniupnpd/1.0 (Status: 501 Not Implemented) 8080 = (SIP end point; Status: 501 Not Implemented) and same as the above for ports 8081 & 8888 & 5540.
All of those SIP ports were just HTTP servers that looked exactly like the customer version you see on http://10.0.0.1/ , except that my admin pass didn't work on it (tried the defaults too, plus some guesses).
When I went to https://172.16.12.1/ it redirected me to /cgi-bin/status_cgi, which contains a link to /cgi-bin/tech_support (which redirects to /cgi-bin/adv_pwd_cgi).
So maybe you could try all of that to see if your TG862G works the same :-)
P.S. I tried the password of the day thing but the seed must be different on this one, and the SNMP thing doesn't exist on any of these webservers.