Setting APN failed !

Dears
In order to benefit my private APN network, I’m using Mikrotik RB951 + Dlink 3g Dongle and It works properly.
The SimCard gives static IP and the PPP-out connection is partially stable. Unfortunately, the connection will be lost after a while and when I checked my router board, I saw the following message.
M1.JPG
Trying to disable and enable of ppp-out connection was useless and faced to the following message.
M2.JPG
would you please tell me how can I solve this problem ?

cheers