@huuanito 

Please reflash the latest firmware on the router or upload the firmware it already has.

Leave the router in the default settings, and test again.

The hardware NAT is known to cause issues with slowdown, but not network connectivity.

@Tony  you may not have noticed but it's already at the latest firmware... when this issue first started that's the very first thing I tried.

and of course it will work with default settings since AFAIK one of those is 'hardware NAT on' so that won't tell anything.

OTOH maybe I misunderstand what you mean by  "Leave the router in the default settings," especialy since AFAIK default settings won't even have guest wifi running, but as I say, maybe I misunderstand what you mean.

thanks

@Tony  here is where I got the idea that hardware nat might be the problem:

https://superuser.com/questions/1170183/archer-c7-ac1750-what-is-the-purpose-of-hardware-nat

in the accepted answer point 2:

" If you do use hardware NAT and experience problems with dropped connections or slowness, try disabling hardware NAT. If that solves your problem, keep it off."

So from that I'd say that hardware nat just might cause the issue, dropped connections,  I described...

Also the issue I'm intersted in is not the dropped connections, I have a workaround for that:

I don't care if I have harware NAT on as my internet connection speeds are all below 100megabit/s, the issue I have is that which is in the subject line, when it's off guest wifi can't reach WAN. For me the dropped LAN connections when it's on aren't the problem, that lack of guest access to WAN when its off is. 

thanks.

BTW just in case tp link wants to reproduce the problem my guess as to when it started is around when I added a few extra devices to  my network. Now have around  8 on wifi and 8 on LAN 

@huuanito 

I can forward this to see what we can find for you.

@huuanito 

Our testers were able to see the same issue you are coming across. With respect to a fix I am not sure about since the router has been end of life since Aug 2015. 

How long have you had the router for?

@Tony 

we purchased the router from the UCSD bookstore on 3/4/2015.

Guess they sold us a very soon to be obsolete model :( :(

tks