Router – How to setup route to gateway on different subnet with MikroTik Routers

We have the following setup which I need to get working:

• Location A:
  • Subnet 1:
    • Network: 192.168.1.0
    • Mask: 255.255.255.0
    • Default Gateway: 192.168.1.1
  • Subnet 2:
    • Network: 192.168.2.0
    • Mask: 255.255.255.0
    • Default Gateway: 192.168.2.1
• Location B:
  • Subnet 3:
    • Network: 192.168.3.0
    • Mask: 255.255.255.0
    • Default Gateway: 192.168.3.1

The default gateways of location A and B are connected via a VPN and ONLY route subnets 1 and 3. I cannot change the config of these gateways.
What I need to achieve is, that traffic going to a public subnet, e.g. 193.197.0.0 is routed through the default gateway of subnet 2.

What I did so far:

• I added a MikroTik router at location A and B which is configured as follows:
  • Location A:
    • IP: 192.168.1.254, 192.168.2.254
    • Route: 193.197.0.0 -> 192.168.2.254
  • Location B:
    • IP: 192.168.3.254

Now, for location A and subnet 1 this works fine. However, I fail to setup the MikroTik at location 3 properly. I tried:

• Route: 193.197.0.0/12 -> 192.168.1.254 (that does not work)
• Setup IPIP-tunnel between the two MikroTik and route 193.197.0.0/12 -> (IPIP-tunnel)

Background: clients in subnet 1 and 3 need to access a certain website. However, the website can only be contacted through the gateway in subnet 2 (some high security stuff).

I'm pretty sure that this scenario can be solved with these routers but I need your help! Any ideas?