RB411U + MC8790 --- 3G(UMTS) and IP problem

Hi;

After 3G connection, passing almost 10 minutes and 3G connection is going down spontaneously but RB411 is still holding former IP and 3G connection is comming again in 1 minutes (3G drop is not comming from ISP, problem from RB411U, it is losing connection). RB 411U is taking new IP after new 3G connection ,double IP is causing IP conflict in this time and Internet is stopping.


Why is RB411U holding old IP ?
Why is 3G connection dropping continual by 10 minutes?

[admin@MikroTik] /ip> address pr
Flags: X - disabled, I - invalid, D - dynamic 
 #   ADDRESS            NETWORK         BROADCAST       INTERFACE              
 0   ;;; default configuration
     192.168.0.1/24     192.168.0.0     192.168.0.255   ether1                 
 1 D 188.38.80.243/32   188.38.80.243   0.0.0.0         3G                     
 2 D 188.38.91.51/32    188.38.91.51    0.0.0.0         3G                     
[admin@MikroTik] /ip>