Access Point connected to the OpenWRT router disconnects randomly

So my current setup is as follows:

ISP ONT [LAN] -> [WAN] TP-Link C6 Archer C6 v3.2 (OpenWRT) [LAN] -> [LAN] Archer C20 AC750 (Stock)

So everything except IPTV connects to the C6. I am using C20 as an AP only (DHCP turned off) for extended WiFi coverage. Now the issue I am facing is that the C20 randomly drops off the internet access completely and the Internet light turns off. If I disconnect from it and try to reconnect the WiFi back it freezes on Obtaining an IP address and never connects. Now if I connect to the C6 router's WiFi, the internet is perfectly fine. Also, the C20 seems to be missing from the connected devices list in the C6's config page.
The only workaround is to reboot the C20 router and everything is back to normal again.

A few things that I have tried are allocating an infinite IP lease for the IP address of the C20 in C6's config and also switching between the WAN and LAN ports of the C20 but nothing helped. The problem as far as I can assume lies somewhere in the OpenWRT config but I am unable to troubleshoot it.

P.S This issue never occurred earlier when I used the C20 as an AP with the ISP ONT directly.

Need your help folks. Thanks.

In other words you have an issue with TP-Link's stock OEM firmware, not OpenWrt - which makes it somewhere between hard and impossible to provide advice here. It's quite possible that TP-Link didn't envision their device to be used as AP, and some of its userspace expecting to own the default route - and maybe going havoc if that expectation isn't met, but that would be something to be debugged on their sideā€¦

But it wasn't the case when the OpenWRT router was not present in between the ISP router and the TP Link AP. Never had any issues then.