[Equip] Problem with Arris TG862

My neighbor has an Arris TG862 in his garage and it provides telephone and wireless service. (The original installers did not run any Ethernet!). He's using a USB Wi-Fi adapter on a PC and it that seems to lose signal from time-to-time (his iPad never drops off his connection even when his computer "can't connect to the Internet"). He does not know the router passcode! I can get to 192.168.100.1 on the router to see status, but am rejected at 192.168.0.1. What advice do you have about proceeding? (I've mentioned the possibility of having Comcast run Ethernet because it looks there's only a short run from the garage to where he has the computer with the adapter, but he's reluctant to do that at this point.) Thanks.