Router with fail-over

I have a bit of a "which one do I go for" situation here,

Currently have a Vigor 2820 with ECI FTTC modem on infinity. We can no longer utilise the fail-over as the ECI is plugged into WAN2, which is the WAN connection that is fallen over to from ADSL usually.

I'm looking to purchase a router with double or triple WAN that can take a 4G dongle/sat' connection.

I ask on here hoping that someone will shout up with experience with such a router and recommend one.
Currently I'm stuck between a Billion and the newer Vigor but unsure if they are the best choice?

The need for fail-over is running IP telephones in an office and servers with remote access.
No need for any special VPN or wifi as I will hang a WAP off it.

Can't you put a 4G dongle into the USB port on the Vigor for the failover? It is certainly listed as an option on my 2830n although I have not used it.

I use this at two sites and it works well (on Draytek 2850 and 2860 series routers). I don't need huge speed so a 3G Huawei E3131 is good enough.

I could insert a 3G or 4G dongle into the USB port but the 2820 can't be configured to fail over to it once WAN2 is in use as the primary connection.

Asus RT-N66U or similar?

They specifically have 3G wan2 failover as an option.

(I have an N66U but don't have a 3G dongle)

J