What is the reason for the loss of the Internet on the repeater ASUS RT-N12?

Question

Iron

Good and bright day!
Available:

1. Router ASUS RT-N56U - distributing Internet from a 4G modem.
   
2. Router ASUS RT-N12 - working as an amplifier (repeater).
   

Essence:
Since the signal from the 56th was not enough to a certain point, the 12th asus was purchased and configured according to the instructions, as a repeater. And exactly what is happening? - We turn on the repeater and the Internet is, literally 5-30 seconds, at a very good speed. Then the internet goes down. The distribution disappears from the 12th, while the rest of the devices connected directly to the 56th have no. It feels like the 56th cuts off the 12th from itself.
Question:
What could be the reason and how to eliminate it?
Thank you.

Answer 1

Won. The reason was simple - the N12 did not have enough power to transmit data. It was cured simply - I moved N56 to a more visible zone for the 12th.

Answer 2

perhaps in the main router there is a Rogue AP detection, because the extraneous point is blocked.
try the wds with ap mode (well, if there is of course such a thing)