How to make a mechanism for transferring traffic from a LAN network to a 3G network and vice versa?

Question

Computer networks

Good afternoon.
Perhaps someone has already solved this problem.
There is a system that runs on two servers geographically located in different regions. Between these servers there is a constant exchange of data. The main communication channel is a local network. In addition, there is a backup communication channel (3G), which, let's say, is in a hot standby, i.e. always connected, but only used when the LAN goes down. There is a task in the system (windows server 2008 r2) that pings the local network every 10 minutes: if the ping fails, then all applications are forcibly terminated and raised again on the IP addresses of the mobile network (static IP addresses are used, the VPN network is up). Accordingly, on the other hand, both networks (LAN and 3G) are always also raised.
Actually the question is: How to make a manual transition mechanism (transferring all traffic) from a LAN network to 3G and vice versa. In this case, of course, it is necessary that access via RDP does not disappear. Manipulations with the physical shutdown of ports (albeit by software) are also not considered.

Answer 1

Your task is as follows:
1. Provide two operable channels, one main, the second backup
2. Raise the transport networks (or network) between sites
3. Set up switching routes to the transport network or changing the channel for the transport network
Now tell us how and what points you decide? Just show your configs, because you can figure out how to do it all on an abstract cisco, nix or mikrotik in five minutes.
By the way, you can take two less stupid (God forgive me) Dlink routers with USB ports for 3g whistles on both sides and organize a tunnel with their help (leave auto-switching on them).

Answer 2

if the ping fails, then all applications are forcibly terminated and re-raised on the IP addresses of the mobile network (static IP addresses are used, the VPN network is raised)

Why is it impossible to raise loopback interfaces with the required addressing, "bind" applications on them and already manage routes to them (let traffic either through 3g or through LAN) in any convenient way (automatically, with scripts at the click of a button)?