This issue just cropped up on the latest firmware for my ROG GT-AC5300 router. Firmware version 18.104.22.168.384_45717. I have never had this issue in previous versions. If I reboot the router it works fine for a couple of days and then I cannot see most of my devices that are Wi-Fi connections. From the router I can ping laptop A and laptop B but laptop A cannot ping Laptop B. Either laptop can see the wired computers on the network, i.e. the server and desktop computers that are wired. I am going to revert to the previous version to see if the problem disappears (which I expect it will) as I never had it before this latest update.
I have had this issue since I bought the router several years ago. In fact, this is the issue that forced me to setup the router to reset itself weekly so it wouldn't go into this weird state, but then I also discovered that the reboot functionality doesn't always work either; however, automated reboots once a week has made this a very rare occurring thing on my end... I would recommend you do the same. I set my router to reboot every Monday at 05:00