Our network across two offices, separated by 5kms, joined by airFibre5 WAN link, which looks just like a cat5e cable.

We have a billion 7800nxl router at one office, with two links to internet, ADSL and USB 4G dongle, which turns on when the ADSL goes down.

At the other office we have a Cisco SOHO cable router, with a super fast link to the internet.

I'd like to configure the network so that internet traffic from both offices goes first via the cable modem at office2, then, when that's down, the ADSL router at office1, and if all else fails, the USB Dongle at office1.

And of course, as services come up again, back to fastest available.

Question 1 : Is this do-able at all?
Question 2 : Can anyone advise on how to go about it.

I've modelled the entire network in GNS3, and have it all working .. just wondering how/what to configure to satisy above requirement,

I can't see how I can add multiple gateways on the workstations/servers/printers, and assume i'm going to have to do something with OSPF configuration at one or both routers.

Any advice appreciated

Grif

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You can't have multiple gateways on hosts. Or you can, but it isn't going to do what you want.

Multiple gateways = multiple default routes with the same metric. Faced with two (or more) identical routes with the same cost/metric, TCP/IP stack will do round-robin load balancing across the gateways, which is hardly ever what you want.

If you want the gateway to choose the best available route, a routing protocol should do the trick. If you need to handle a scenario the default gateway in a network may become unavailable, you need to use a first-hop redundancy protocol like HSRP (Cisco proprietary) or VRRP.

I guess you could make the entire deal a lan and then let gateways be used by metric.

There are I'm sure many many more ways.

Fail over virtual machine running a distro. Dedicated hardware failover.