Some additional logs which may provide some insight:  

@Carbo 

Not having all locked channels and also having crit 3/4 errors relate to the internet signal that is going to the modem. Since the internet signal is not reliable the modem loses the internet, and in turn, the router loses it as well.

Usually, a tech will inspect the cabling and replace if need be. It is important to note or inform them of those specific errors or else the ISP may just see that you have internet so everything is fine.

@Tony Thank you!  I spoke with Comcast support today. The young woman was polite, but all she did was reboot the modem and said it looked fine.  I insisted on a tech coming out and I was told someone would contact me.  Unable to make appts any more over the phone with Comcast.  

@Carbo 

Ya, there is only so much that you can do.

Do you happen to have a spare router lying around? If so, you can put that temporarily, and monitor it, if that does not ever go down that then can point toward the TP-Link as being the cause.

If you do not have another router, make sure your AX50 has the latest firmware, there was a recent update released on the 10th of this month. If you have not updated since I would get that done. If you happen to have a spare Ethernet cable, swap that out, if you do not have another Ethernet cable that is ok. You don't need to buy another one.

I'm having the exact same issue. Started in the last week or so. I also have Comcast. However, I have AC5400X

Spent an hour or so on the phone today with Comcast.  The polite woman on the phone assured me she was a tech expert, and started out with the usual:  rebooting modem, router, and laptop.  Had me fuss around with the CMD prompt, flushed the IP, etc.  She insisted all the numbers on her end were in the proper range and the issue is with my equipment.  I asked her "What about the log files, peppered with T3 and T4 timeouts?"  She said she saw that but didn't think it was the issue.  Her advice: I need a new modem.  I asked her to send out a tech but she refused, telling me my internet is up and running.  I explained I have internet but the problem is that it cuts our randomly 4-6 times daily and then reconnects on its own a few minutes later.  Comcast's policy is no tech unless the internet is down when phone support runs a check.

@Carbo 

Why don't you disconnect the modem next time you call them? 

@noxxy  LOL.  Good idea.  But when the tech shows up at my door and he sees the internet is up and running, he'd probably be out the door in minutes.

@Carbo 

Doing these things can also help to simplify if it is ISP or your side.

First be sure your cable is 100%. buy a new one if thats what it takes to check and have a backup. I made mine but  many failed so that is the first rule.  (yes my failed cables still worked but caused slow speeds)

I have found if your having issues be sure you turn off a lot of those things we wanted this routher for to begin with.

disable all your QOS or priorities. 

Turn off IPV6. ( if on be sure you configured it properly)

(I did this the other day having 3 machine in priority and my service speed dropped to a crawl. )

There is a lot of changes in the internet happening and even though we have astrong grasp of our older gear there is difference in this newer gear that may not play well till you learn its ways.

Even if you have say a Gig service a Cat5e from the modem to the router/wifi will still work for gig service.  Check that your cable incoming from the drop is good and to the box  that a good ground is in place and then filter is good to your modem.  If you don't use any other splitters of even need a splitter in the box, have them go direct to the box to modem  this removes and other signal issues.  Then ends can cause signal issues even if it is one tiny wire touching.

I have a mix of cat5e and cat6 on my network and when your provision is correct it all works great.  I have made a fool out of myself more then once.

Oh and at worst plug from modem to PC and see if you connection up/downstreams are right.  Thsi can help ISP faster.

Just a view that some ISP's will want you to isolate. Just uncomplicate everything till you get a solid connection with proper speeds.  

Good Luck!