I have an Airport Extreme running the latest version of the software. All settings are pretty standard although I may have overriden the wifi channels. I have the 2.4 network and the 5ghz network with different names. Some devices in the apartment are on 2.4 and some are on 5 depending on what they are etc. I have a 250mbit cable connection into an Arris router attached to the Airport Extreme.
I have a NAS, a wifi printer, several Samsung TVs and rokus on the network. Depending on which network certain devices ( ipad, iphones, ps4, TVs, ROKU ) are on compared to what the NAS, printer, rokus, tvs are on they can't see each other. If I disconnect and then reconnect wifi on the ipad/iphones/TV/rokus ( multiples of them ) then I'll see the missing devices. Almost like the 2.4 and 5ghz are not "sharing" the "routing table" or not sharing the network and making everything regardless of network available to all.
I've used plenty of DD-WRT and the like on devices over the years and have never had this issue. I dumped all of them to go "simple" and just use the Airport Extreme going forward, but it's driving me crazy when I have to keep resetting or rebooting things to get them to see each other, which should not be the case.
There is limited config options on the Airport Extreme, so not sure there is anything I can check or should check. Note that 1 device is only ever connected to 1 network ie. the printer cannot jump between bands. Same with all the other devices.
The NAS is plugged directly into the Airport Extreme. The TV is on 5ghz and sometimes I can't see the DLNA broadcast on the TV. If I hard reset the TV ( pull power ) and then re-plug it'll pop up once it reconnects to the network. Same deal with the Printer but that is on wifi and phones printing are on wifi.
Apologies for the redundancy in my explanation, I wanted to explain some scenarios as best as possible.