I had the same dilemma last year when some devices on the network will just drop randomly and it will even freeze my internet connections. This includes the access points, Smart tv and streaming boxes, cameras, laptops, desktops, wireless handheld devices including NAS and printers.
I have check all wiring thru out the house one by one including replacing some switch, RJ45 jack and wires to routers as access points. I also used an Asus router to handle as the main router using several brands of none manage switches. It is really a hair pulling experience.
It took me several months to figure out those random drop outs. The solution I did was to build a new itx with Kaby Lake pc dual core 8 gigs memory and bought a used Intel 4 port server nic and run Pfsense. I also got rid of all the switch and bought Ubiquiti Unifi switches and access points.
It's been running 24/7 for almost 10 months none stop and not a single drop out. The only time the router gets rebooted is for a making big configurations and upgrading the firmware. The ghost are gone!
Since I had several custom water cooling parts lying around from my previous build, I water cooled the CPU including the network card and I was happy as a clam.
https://imgur.com/nHzzU8F
Also make sure your modem is in bridge mode.