How can I restore the tablet in this case (changed the rights to / system /)?

Question

Android

There are 2 identical tablets on hand (we denote A1 and B2).
Both are some Chinese noname with custom firmware based on Android 5.1
A1 - root was obtained, after which the rights to the / system / folder were changed. The tablet goes into bootloop.
fastboot does not flash recovery (access denied for writing), adb writes device unauthorized.
B2 - worker, got root.
How can A1 be restored (using B2 resources or just)?
While I came across the advice to close the pins on rom memory - but I did not quite understand the full algorithm of actions for this method.

Answer 1

1. Tried to unlock A1 in fastboot mode before writing recovery?

fastboot oem unlock
fastboot flashing unlock

2. Try to download the recovery image without firmware:
fastboot boot recovery..img