I've been testing the 2920n 's 3G failover feature for the last week and was pretty pleased with it.
However I'm running into problems when the router's main WAN interface is online during a longer period (24h+).
If i then pull out the modem that's attached to WAN1, the router tries to bring WAN3 online but somehow fails...
When i then restart the router (using the power switch on the back) it brings WAN3 online just fine.

Has anyone experienced the same problem? Is there any solution?
Is there some way to have the router reboot once on a daily basis?

I've seen that with 3G before back when I was using it as a main internet connection with a 2820 but it depends on the modem really - are you using the 3.3.7 firmware on the router?

If it still fails to detect with that, I recommend getting syslog output from the router when that happens and contact draytek support so they can look at it.

I'm using the 3.3.7 firmware. The modem is a Huawei e367. Once it dials it seems to work ratherwell. It only seem to fail to dial *sometimes* at other times i can disconnect the main wan and it behaves just fine.

This is what the logs say when 3g refuses to dial...
1662011-11-03 17:39:16Nov 3 17:38:43DrayTekVigorWAN3 PPPoE ==> V:1 T:1 PADT ID:0
1662011-11-03 17:39:16Nov 3 17:38:43DrayTekVigor[3G]Send default init command AT&FE0V1X1&D2&C1S0=0 #1
1662011-11-03 17:39:16Nov 3 17:38:43DrayTekVigor[3G]Already start (1) ... wait
1662011-11-03 17:39:16Nov 3 17:38:43DrayTekVigorWAN1 DHCP ==>XID = 0x788bb9e1, Client IP = 0.0.0.0, Your IP = 0.0.0.0, Next server IP = 0.0.0.0, Relay agent IP = 0.0.0.0, Option:
1662011-11-03 17:39:19Nov 3 17:38:46DrayTekVigor[3G]Already start (1) ... wait
1662011-11-03 17:39:21Nov 3 17:38:47DrayTekVigorWAN1 DHCP ==>XID = 0x788bb9e1, Client IP = 0.0.0.0, Your IP = 0.0.0.0, Next server IP = 0.0.0.0, Relay agent IP = 0.0.0.0, Option:
1662011-11-03 17:39:22Nov 3 17:38:49DrayTekVigor[3G]Already start (1) ... wait
1662011-11-03 17:39:25Nov 3 17:38:52DrayTekVigor[3G]Already start (1) ... wait
1662011-11-03 17:39:29Nov 3 17:38:55DrayTekVigorWAN3 PPPoE ==> V:1 T:1 PADT ID:0
1662011-11-03 17:39:29Nov 3 17:38:55DrayTekVigor[3G]IPCP fail #1
and then in blocks it does
1662011-11-03 17:42:26Nov 3 17:41:48DrayTekVigor[3G]Change AT command type 1
1662011-11-03 17:42:26Nov 3 17:41:48DrayTekVigor[3G]Send default init command AT+CFUN=4 #14
1662011-11-03 17:42:28Nov 3 17:41:51DrayTekVigorWAN1 DHCP ==>XID = 0x788bb9e1, Client IP = 0.0.0.0, Your IP = 0.0.0.0, Next server IP = 0.0.0.0, Relay agent IP = 0.0.0.0, Option:
1662011-11-03 17:42:29Nov 3 17:41:51DrayTekVigor[3G]Already start (1) ... wait
1662011-11-03 17:42:32Nov 3 17:41:54DrayTekVigor[3G]Already start (1) ... wait
1662011-11-03 17:42:35Nov 3 17:41:57DrayTekVigorWAN3 PPPoE ==> V:1 T:1 PADT ID:0
1662011-11-03 17:42:35Nov 3 17:41:57DrayTekVigor[3G]Already start (1) ... wait
1662011-11-03 17:42:37Nov 3 17:41:59DrayTekVigorWAN1 DHCP ==>XID = 0x788bb9e1, Client IP = 0.0.0.0, Your IP = 0.0.0.0, Next server IP = 0.0.0.0, Relay agent IP = 0.0.0.0, Option:
1662011-11-03 17:42:38Nov 3 17:42:00DrayTekVigorWAN3 PPPoE <== V:1 T:1 PADT ID:0
1662011-11-03 17:42:38Nov 3 17:42:00DrayTekVigor[3G]Modem stop
1662011-11-03 17:42:46Nov 3 17:42:08DrayTekVigorWAN1 DHCP ==>XID = 0x788bb9e1, Client IP = 0.0.0.0, Your IP = 0.0.0.0, Next server IP = 0.0.0.0, Relay agent IP = 0.0.0.0, Option:
1662011-11-03 17:42:47Nov 3 17:42:09DrayTekVigorWAN3 PPPoE ==> V:1 T:1 PADI ID:0
1662011-11-03 17:42:47Nov 3 17:42:09DrayTekVigor[3G]IPCP fail #14
The funny thing is: i just tested disconnecting and reconnecting the USB modem and then it dials without a problem.

If you've got syslog then send that to draytek support, the modem isn't initialising properly for some reason, maybe the driver stops working after a few hours?