@malada 

Did you put the Gateway (Xfinity's modem/router) in Bridge mode? 

See https://www.xfinity.com/support/articles/wireless-gateway-enable-disable-bridge-mode

In the long run if is less expensive to have your own modem and router. Be sure to get an approved modem that Xfinity supports and I recommend you get one for a higher speed tier. They have a list on their web site.

Be sure to power cycle the gateway, router and PC (all off, power up the gateway, wait, then router, wait, then the PC).

I always test using either a third party speed test or the Xfinity speed test.

@ArcherC8 That did it. I put it in bridge mode. Then I did the speed test but still got 115mbps. Then I rebooted the modem and router. After they were up I did another speed test and now I am getting about 450mbps.

I upgraded our speed and got a modem/router from xfinity free for 1 year. I decided to get their equipment because our internet service has been spoty. They keep on blaming my equipment so I got theirs. I suspect I will still have the same problem so now they don't have excuses anymore. So then before the one years is up I will send the equipment back. Should the spoty internet go away then it is indeed my modem (Motorola SURFboard SB6121) in which case I will replace it. Btw, I didn't know they have an approved list of equipment. I have to check that out. 

@malada 

Glad that worked,

Free for a year... I would have done the same.

The approved devices vary by location. See https://www.xfinity.com/support/articles/list-of-approved-cable-modems

They control the software on the modem.

When you get one, get one with a lot of up and down channels as when I did that, my network experience improved. Also look for DOCSIS 3.1 and IPv6. Look at the higher speed tiers.

@malada 

Another thing to check on their device is the channel settings and logs. 

For channels:

make sure all channels are locked.  Locked is active.  

Make sure no channel is taking excessive errors.  Some errors are common but a lot is not

Power levels -12 to 12 

SNR 30-50

For the Logs:

Make sure there are no T3 or T4 timeouts.  Those indicate a line issue and almost always require a maintenance dispatch to plant to resolve. 

@ArcherC8 

I am thinking of either the Motorola 8600 or Motorola 8611. Maybe they will have a black Friday sale...

@malada 

I think either would be a great choice. I would lean toward the MB6811 if you are even thinking of gig speed of at a great price plus it should have a longer life span as it is newer.

I have the Motorola SB8600 and have been happy with it. It has been trouble free. I have 200 service and most times my speed checks are in the 230-238 down range and 11-12 up.

I have IPv6 set up.

I get 32 channels down and 5 up from Xfinity in Central Fl.