Set source IP for client

I have multiple small, disparate, clients that I provide Disaster Recovery backups for. My problem is that most of them have the same private IP address hidden behind a NAT firewall. I put in Ubiquiti EdgeRouters for these clients, and can create VPN tunnels pretty easily, but beyond that the firewalls get over my head. I would love to be able to bind the urbackup client to a secondary IP address on the server and create the vpn tunnel using that IP network.
I don’t find anything like this, and I would really like to not implement additional pieces, ie proxy setup. These are small business with 1-3 servers at max, and budget very little for actual IT services.
I am working with Ubiquiti to setup source NAT and dest NAT, but it starts to get complex pretty quickly, at least for me…

Any ideas?