Thanks for all responses
And the suggestions how to create a better set-up.
But I don't think that will give a solution to the not working Internet via 5GHz, or does it?
As it should work out-of -the box...
Unless you want 2 completely different, isolated and firewalled subnets in your network where devices connected to router A can't talk with devices on router B, then this is how it works "out of the box". If you have 2 routers in your network, one must to be configured as the "master router to the internet" and the other must be configured as a "wifi provider for the local LAN (aka dumb AP)".
Why do you want to give up 20% of your ports on a dumb access point? I mean I prefer to hook-up 4x LAN + Wifi devices. I use it in the livingroom near my TV/radio/receiver/android box. And now a days they all want Internet via RJ45 (I prefer cable above wireless access)
I sold my trouble Netgear R7800 and got me another one (2nd hand online) and this worked out of the box with OpenWRT and 5Ghz. Although it seems to contain same hardware, there is something that prevents it to access Internet via LAN or WAN port