MTR device gets thrown back to the login page after clicking on the "Sign-in" button.

TheGrahamWalsh 

As the A20 is an Android device, the solution was to allow the Android Device Manager via the Endpoint manager for both company owned and BYOD devices: https://endpoint.microsoft.com/#blade/Microsoft_Intune_Enrollment/RestrictionMenuBlade/

There's also an arbitrary limit on the number of devices an account can enroll (15) these records are apparently only deleted once every 30 days or so.

There's another menu blade somewhere in the endpoint manager which shows device

enrollment failures, where you can search for the account get a clue what the error might be, though I do not know how to navigate to it (before you could access it from the home page)

Hope this helps anyone with the same issue.

My recommendation is to pull out the device logs from the Teams admin center and and vendor specific logs, and contact the manufacturer of the device, as the error codes are not user friendly, and , evidently, Microsoft provides no end user error codes to distinguish between the dozens of errors which can happen during enrollment 
(Such as conditional access, enrollment restrictions, Azure resgistration restriction, Intune Registration restrictions, device enrollment limits) 
as in my case the MTR device just looped back to the login screen.