Problem:
Aug 2 00:04:46 OpenWrt daemon.info pppd[9652]: Serial connection established.
Aug 2 00:04:46 OpenWrt daemon.info pppd[9652]: Using interface 3g-WAN
Aug 2 00:04:46 OpenWrt daemon.notice pppd[9652]: Connect: 3g-WAN <--> /dev/ttyUSB0
Aug 2 00:05:13 OpenWrt daemon.notice pppd[9652]: Modem hangup
Aug 2 00:05:13 OpenWrt daemon.notice pppd[9652]: Connection terminated.
Aug 2 00:05:14 OpenWrt daemon.info pppd[9652]: Exit.
Aug 2 00:05:14 OpenWrt daemon.notice netifd: Interface 'WAN' is now down
Solution:
config 'interface' 'wan'
...
option pppd_options 'debug noipdefault'
+ SET CORRECT TIMEZONE!