How to set up simultaneous Wi-Fi as WAN and Wi-Fi AP

…and a few days later it’s happening again. SSIDs disappearing at random. I now only have the one SSID, but it’s only visible on 2.4GHz (even though it is setup for both). All clients have connected with 802.11ng (Nobody on 802.11ac). WiFi WAN is connected to 2 different SSIDs on the remote router - 5GHz and 2.4GHz (Same router).