You get the red sign if the tablet is charging at a diminished rate.
The OEM charger for my tablet says 2.1A, 5,3V.
I've gone round and round pointing out the diminished capacity of USB 2.0, 3.0 and OTG charging.
(0.5, 0.9 & 1.5A respectively)
Use a dedicated wall charger with 2A charging and you will not get the slash, if your cable is good and the port not littered with debris.
Chargers wear out.
Cables do too, from the constant flexing.
If you have tried both, get a magnifying glass and a strong light and look in the charging port.
Phones get pocket lint and 'stuff' packed in there all the time.
Given that Monica has battery problems already from running her tablet down until it shuts off, I would agree with Dale that it is a battery issue more than a charging issue.