Wired and wireless networks cannot access each other

ATT Uverse customer for many years.

Modem 5268AC FXN tied to an 8 port trendnet GB dumb data switch.

Using the built in WiFi on 2.4GHz only.  A 2.4GHz Netgear wireless router with fixed IP, no DHCP in the garage simply hanging off a network drop.

Security active, pre-shared passkey, etc.

Fibre to the residence installed around the same time these problems started to occur.

Over the past three months I've had a series of strange problems with devices on my LAN becoming inaccessible.  The printer would work from the desktop PC, but when a print job would originate from the laptop, printer was offline.  It is on a hardwired connection and has worked for years. Go to desktop, call up same job, prints OK.

Couple days later, no apparent problem printing from any device...

Couple days after, needed to print out an email, printer is again offline from laptop using wireless connection.

Etc.

I have a house remote control system that has a user interface for iPhones, iPads, Androids, etc.

I'll reach in my pocket, run the app, it'll indicate that the host isn't available at the IP address.

Ask my wife for her iPhone, run the same app, it works fine.  The next day, the reverse will be true...that is, mine will work, but hers will not be able to access the host.

On several occasions I've trouble shot this problem finding that 1 or sometimes all of the iPhones won't work, but the iPads will.  On one occasion, the only device that would access the host was the android Samsung Galaxy tablet.  No iPhones or iPads.  Next morning, as I prepared to call ATT, everything again worked.

I figured this was a problem with the home remote control, right?  Wrong.  Its a symptom of something else.

In an unrelated chain of events, the printer mentioned in the paragraph above failed and was replaced with a nice HP that was wireless only.  Initially, it worked great.  Install was straightforward and successful on desktop PC and wireless laptop.  48 hours later, the printer was unavailable to any device on the wired network, only to wireless devices!...and that was my first clue.

BTW, all wireless and all wired devices always connect to the internet, just not each other.

It recovered while I was at work and that evening, all was working fine.

Eventually, it failed on a day when I was home from work and available for 'hold time' with ATT.

The technician was courteous, but unfortunately, didn't understand my problem or didn't have sufficient experience or knowledge.

At any rate, he followed the script and did the tests in order and when he rebooted the residential gateway, the problem disappeared...so problem solved, thank you sir, have a nice day.

He also said my modem had failed to update the firmware on recent attempt, did it manually, and was successful while I waited.  This could've been the whole problem...

Anyway,

I am not certain this is a defect in my modem.  I have read and examined my settings and wonder if there is an incorrect setting somewhere.

The IVP6 stuff, which has not been active before, is all showing up on ipconfig in a console window on the PCs.  Running FING on iPhone, iPad when the problem exists reveals ping connectivity to the ATT router, but no other wired device.  Similarly, using Angry IP Scanner on a Winders 10 machine will show wired devices but varied wireless device connectivity.

Sometimes there will be NO wireless connectivity, sometimes SOME devices show up, sometimes all will show up.  

As I began write this, the problem existed and having run these scans again just now to insure the accuracy of my post, the pings have started responding again.

This is driving me nuts.

If you have any insights, I'd appreciate it.

If there's a way to get an ATT someone technical to have a look at my router, it would be great.

I guess I could just demand a new one, but I'm not convinced its defective.  I think there is a setting that has been changed, etc.  Perhaps a network problem that is tied to implementing IVP6?  I'm technical, but not a network guru.