@rctp 

I have the light off on my router as that is what I prefer.

If you get a new modem, get one on the ISP list with a higher speed tier as it will be better and should last longer. Cost difference is not that much. Look for the most up and down channels as that improved my performance when I upgraded a years ago. Xfinity had also raised my speed several times since then.  I am at 500 now with a Motorola modem with 32 down and 8 up and max speed of 1000 as I recall. Xfinity is giving me 32 down and I think only 3, 4 or 5 up. I suspect my modem is getting near end of life. 
 

If it is working, I would not change it unless it would be a free return for the router
 

Also have you tried a factory reset on the router? Also be sure you are using the power plug that came with the router. 

  @rctp   

My present Xfinity speed tier is 170 down and 20 up. I am considering these modems, Motorola MB8611 2.5Gp and ARRIS SB8200 1Gps; hoping the unlit Internet/Globe icon light will turn on.

The router, TP-Link AX1800 AX21US, started a very degraded and worthless connection to three stationary devices which worked perfectly. I haven't been able to fix it by disconnecting/connecting, or resetting.  Since my present ARRIS SB6190 modem, is no longer supported by Xfinity, I have ordered the Motorola MB8611 modem which hopefully, will solve the two problems. Delivery is not until August 10th. Amazon is getting slower and slower.

  @rctp 

If it were me and the router performs fine except for the lights, I would live with it as it appears it may be your Modem or ISP since you used two different routers. Also I have my lights off as that is what I prefer.

I think you should consider getting a newer modem at some point. Do get one that is faster than your current speed tier. My current speed is 500 and the modem I got was a Motorola MB8600 which was one of the faster ones when I got it as my speed was like 200 back then. Now it is close to the end of life. My old modem had only a few channels up and down. The 8600 has 32 down and 8 up. Xfinity is giving me 32 down and 5 up at my speed tier in this area. It gave me more uniform speed and better speed with those extra channels. Think of it as more lanes in a highway.

Just a thought, make sure you are using the power plug that came with the router instead of the one from your old router.

  @rctp  I have ordered Motorola MB8611 and will receive it on the 10th of August. 

  @rctp 

I would expect that modem to handle your needs for many years.