How to Disable AP Isolation on Linksys WRT 1900 AC v2 with Stock Firmware for LAN and Wi-Fi Device Connectivity?

I am having trouble disabling AP isolation on my Linksys WRT 1900 AC v2 router with stock firmware. I have two devices that both have internet access - one is connected to the LAN and the other is connected via Wi-Fi. Despite both devices being connected to the same router, they can’t ping and connect to each other. Can you help me find the configuration to disable AP isolation? Both devices have internet access coming from the same router.

Question by cyberquarks on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.