LAN traffic not working

I am an aspiring prosumer and am trying to learn the ins and outs of networking as I'm going along, so please be kind. That said, prior to switching to my Gold SE in router mode, and an AP7 ceiling unit, I had no problems with my devices speaking to each other, now I am constantly running into trouble with local connectivity.

I frequently am unable to airplay from my iphone to my homepod minis. Sometimes the homepod minis show up, most of the time they don't. I had one Macbook Air that would wirelessly backup to another Mac desktop for Time Machine purposes. Now, the laptop can no longer see the shared folder on my desktop. My Macs can no longer find my printer to print wirelessly.

I try to ping my various computers and devices and 90% of the time I get timeout error messages.

Things that I have tried so far:

Everything is on the same SSID

Everything is on the same subnet

I have not set up any VLANs nor turned on VqLan

mDNS Relay and SSDP relay are turned on

I have devices set to their respective users, but have not implemented any rules about limiting traffic between users.

I've turned off ad block, advanced threat filtering, region blocking, content filtering, device active protect, Unbound, NTP intercept, I'm not using a VPN...

Ultimately, I reset my Gold SE to factory settings, and then re-created my SSID and did nothing else apart from re-add my devices. And all the above problems persisted. Is there a setting I'm missing somewhere that is preventing reliable LAN traffic? 

I really want this to work, but if anyone has any bright ideas, and would be so kind as to speak in laymans terms, I'd appreciate it.