Arris SBG10 won’t connect to Xfinity internet

I recently moved in to a new apt and brought my SBG10 modem from my old apt. It was working prefectly without any issues. I activated it again through Xfinity app but have connection trouble despite many attempts to reset it and reactivate it. It won't connect to internet and many app and websites are not available. Smart TV won't connect to it at all. only some app, like Youtube on my iphone, are working on the network. All the modem's lights lit steadily. Xfinity live chat didn't identify the problem on their end and sent a technician to figure out the problem, but the guy who came out was unprofessional and lacks technical knowledge and experience. He did not help to fix the issue and simply said that I need to re-configure the modem. I logged in and check the modem settings, but couldn't figure out what setting I need to change. I would greatly appreciate any help. Thanks.