Directly connecting to Linksys works OK all the time?
Try to use C7 as an access point to see what happens. http://www.tp-link.com/en/article/?faqid=417

yes, it works fine if I use only the Linksys

another thing I've tried: set C7 WAN IP to static. Behavior was the same - random lockups few times a day

the most interesting part is, that not all traffic is blocked - for example, If I'm streaming video it might continue to work. If I'm on some webpage, mostlikely navigating thru the page will also work.
but any *new* connections don't pass thru. Seems like the NAT part is locking up or something?
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yesterday I've changed the wired connection to cascade mode - e.g. plugging the wire not into the WAN port of the C7, but into the LAN port;
then disabled it's DHCP service. This way (I think) the C7 NAT routine is not involved, as all devices are in the same subnet, with IPs assigned by the linksys.
well, guess what - more than 24 hours no lockups.

I'll keep monitoring, but if that holds true, the C7 is basically usable just as an AP and not a router

I see this thread dates to 2014, and here it is 2017 and I'm having the same problem.

I have a Archer C7 v2, Firmware Version:3.15.1 Build 160719 Rel.57530n

What is the non-techy version of how to fix this problem? I just want to be able to use the router without having to learn the industry to make it work.