VPN connection as default gateway for LAN

[makbet]

Hi guys
I'm struggling with some configuration for couple of days already.
I've setup my firewall. All up and running.

Let's say firewall's external IP is 1.1.1.1 and VPN's external ip is 2.2.2.2
I have two subnets: 192.168.1.0/24 and 192.168.2.0/24
I have setup vpn client on my firewall and when connected it get's following ip: 10.8.0.10

Now I need 10.8.0.10 to be a default gateway for subnet 192.168.2.0
Subnet 192.168.1.0 via firewall's default gateway.

In another words, when computers from subnet 192.168.1.0 will go to whatismyip.com they should see 1.1.1.1 as their ip
When computers from subnet 192.168.2.0 will go to the same page they should see 2.2.2.2

Whenever I'm trying to add 10.8.0.10 as a default gateway in Network > Gateways it tells me the ip is unreachable even if ebox is currently connected to the vpn. Same is when I'm trying to add the static route I get the same message

So how to achieve what i just described?

[makbet]

Just to help you out guys, I'm looking for using below command in WEB gui

route add default gw 2.2.2.2 eth2