@rctp 

Check to see if you have it turned off  Go to advance settings then system, then led control. 

  @rctp If the Internet light on your Archer AX21 router is off while Wi-Fi and Ethernet connections are functioning, it suggests a potential issue with the router's connection to your ISP. Firstly, check if the modem is properly connected and powered on. Ensure that all cables between the modem and router are securely connected. If the issue persists, try rebooting both the modem and the router. If the Internet light remains off, it may be necessary to contact your Internet Service Provider to verify if there is an outage or if specific settings need to be configured.

  @ArcherC8 Good idea, I checked and is not set turn light off.

  @Tommy2442 Thank you for your reply.

I repeated the checks you suggested including disconnecting the Ethernet cable from the WAN socket and connecting it to one of the LAN sockets using different Ethernet cables and signing into the router controls to Internet Router Negotiation Speed set to 100Mbs Full Duplex.

Spent time on two different days with TP-Link chat support but after a while of trying different things became disconnected in both instances, a frustrating endeavor which led to my returning the unit but, at a calmer later date ordering the same model because of its good reviews. Same problem with the light off and all Wi-Fi and Ethernet connections working perfectly.

I have today posted the problem at Comcast/Xfinity Community Forum requesting that they check the connection's settings. Will get back to this posting to report.

My older router TP-Link C9 started intermittently disconnecting wireless and wired connections, after many years of good service.

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  @rctp 

Be sure the firmware is up to date in the router.

I would suspect the Ethernet cable but you said you changed it. Look at the modem connection and ensure no bent pins in the connector.

power off and unplug the modem for 10 min and then plug it in and power on. Sometimes that forces the modem to check to see if it needs to be updated by the isp. 

is the modem at end of life or no longer supported per xfinity?

since it is xfinity, did anything else change such as remove a tv or change of service?

  @Tommy2442 Thanks for the reply.

Have made all the checks and resets still nothing works.

Have contacted Comcast support by phone and after checking several things, the tech indicated that they no longer have access to this router settings because it's 4 years old and out of their system, I decided to pay more for this router because of all its great reviews but these features are not working, turns me off. I'll save some money and buy a TP-LINK AC 1200, Archer A6 for just $40 which was my original budget, unless you have a better router suggestion.

  @rctp 

I can not tell you which would be the best router for you as everybodys needs and network is different. The advice I have is to buy one from a place that has a good return policy. Test it before the return time is up.

  @ArcherC8 Sorry for the delay, I have been sick for a couple of days.

The firmware is up to date, and the Ethernet cable and pins are good, and tried another cable; disconnected the modem for 10 minutes, and reestablished the internet: The Router's internet light is still off.

My model Arris Surfboard SB6190 is no longer listed by Comcast/Xfinity and is no longer supported. The next Arris Surfboard is the SB8200v2, SB8200, and SB8300.Which I guess could work?

  @ArcherC8 Thank you for the valuable suggestion. I am also constrained to a budget like most people.

  @rctp I am considering keeping this router AX21US and modem Arris Surfboard SB6190 even without the Internet light not turning on. Except for the internet light everything is working perfectly well, I don't see any problems otherwise.  Do you see any adverse effects by keeping this combination? Thank you very much.