Possibly too soon to declare problem solved but appears this error is/was rooted with something gone awry by Apple device migration when I replaced iPhone 6+ with 11 Pro Max in Fall 2019.
Apple device migration appears to “clone” some deep stuff so my best guess is perhaps some vital nugget of device identification data becomes corrupt for whatever Tablo needs to authenticate the device from remote location?
Simple solution. Merely delete then clean download and reinstall app on the new iPhone. I overlooked doing this until technical support mentioned it and since doing so the app remained remotely accessible from the opposite side of the continent for a little over a week. Longer than the 3-4 days it took to fail these past few months.
The next couple of weeks require me to travel constantly so that will be the ultimate test. Fingers crossed, it’s looking better than ever so far!
My thanks as usual to tech support. And if another user suffers this problem after migrating to a newer iPhone it’s at least worth a simple and easy thing to try.