VPN not accessing server after internet drops

Hi guys.

I am having a rather strange issue that I haven’t been able to resolve.
I have a client with an Ubuntu server in data center, however occasionally the route to the server seems to “stop working”.

Here is a basic network layout.

Client side: 192.168.4.0/24 (RB700) with internet passed from a 3G router.
L2TP 10.0.254.4
IP Route to 192.168.100.0/24 via L2TP connection

DC side: RB2100UM with public IP
L2TP 10.0.254.3
Server 192.168.100.1
Local address 192.168.100.2

We have found that after the 3G drops and reconnects, the route breaks and the client is no longer able to connect to their server.
I really hope someone is able to help!

Thanks in advance.