How to access LTE modem via VPN?

Question

Computer networks

There is a vps server with ip 185.188.xxx.xxx
On it vpn l2tp+ipsec with ip on the server 192.168.42.1.
At home there is a Mikrotik router with a usb modem.
Mikrotik is available on the home network via ip 192.168.88.1, a modem with ip 192.168.8.1 is also available on the home network
Mikrotik receives Internet from the modem, the vpn client has ip 192.168.42.10.

The task is to open the vps server via curl to the modem's webmort via vpn.
Cretinism in the network settings does not allow me to do this and ask no one. On the Internet, I did not find anything like this, but what I found remained unanswered. For example, How to smartly forward a port to access an LTE modem through a VPN?
Who will help you figure it out, tell you where, what rules to draw up and prescribe - I'm ready to pay.

Answer 1

Route to 192.168.8.1 through VPN tried?
The main thing is that the settings on Mikrotik allow you to go through the VPN to this address.
I use an OpenVPN server (but I think the logic will be the same), I have 2 subnets:
192.168.88.0/24 and 10.10.10.0/24 - Main.
10.8.8.0/24 - VPN client subnet.
The firewall allows forward to the main network.
The config on the client says:

route 10.10.10.0 255.255.255.0 10.8.8.1
route 192.168.88.0 255.255.255.0 10.8.8.1

This way I can access these subnets through the VPN.
In your case, I assume, there should be something like (on the server):
route 192.168.8.1 255.255.255.255 192.168.42.10