Background - our 2860 is connected to the internet with a slow ADSL connection. We have a Windows Server network where the server is the DHCP and DNS server. Internal ip is 192.168.20.0

We want to use a 4G dongle for a slightly faster VPN to an external site. I have used the ZTE MF823 with a laptop and can get a good dial in connection to the other site (also a 2860).

So I can get the 2860 to use the ZTE dongle in 4G DHCP mode (it does not appear to work with PPP). Can someone explain what '4G DHCP mode' means?

I get so far in the Draytek guide and it then takes you to the router's DHCP section ... but I don't want to use the router's DHCP!

So the 4G connection is 'active' but I can't see how to use it for my VPN - can I? Perhaps I need to give up and get a dongle that would allow a PPP connection?

(the 4G connection shows an ip of 192.168.0.162 - I don't understand where this comes from)

Any help or explanation would be very much appreciated!!

The USB dongle presents a virtual ethernet interface and it's the USB dongle that's providing the DHCP address to the router. I've only ever seen it give a private IP too, but wondered if there might be specific sims or packages where a public IP is allocated.

Ah ... so the dongle is being a dhcp server? It's still not clear from the Draytek guide how to make use of it. The ip address shown is not on the routers subnet and the guide doesn't really explain it.

Interestingly the LAN to LAN vpn 'connects' when I set it to WAN3 (dongle) ... but I appear to be unable to ping / connect across the vpn.

I've put in a support request with Draytek UK so I'll see what happens. If I don't get anywhere I may have to buy a dongle that will do a PPP connection, which I assume will give me a normal WAN connection(?).

Interesting article here http://www.elevendroids.com/2014/06/changing-zte-mf823-4g-modem-ip-address/ about configuring the ZTE MF823. Looks a little involved but I may have a go later!