MODEM/ROUTER TO VPN ROUTER

hi this is my first post, so hopefully i get some help.
I want to set up a vpn router/switch to get access to network from a remote location. This is my configutation.

modem/router some computers are connected directely to the switch part of this modem. (it is a modem, router ans switch all in one) ipaddress: 192.168.1.xxx for the all the computer connected to this m/r/switch.

VPN router and switch: This vpon router is connected to the modem and some other computers are connected to switch portion of this vpn router/switch. However, its ip address is 192.168.0.1 so the computers connected to this router are in the 192.168.0.xxx range.

My problem is that I cannot ping or access share folders or even use remote destop from the computers connected to the modem router/switch to the computers connected to the vpn router/switch

I know I should use the vpn router only instead of the other router; however, that other routers is the a modem too. that is the modem I got from the ISP. I don;t see any way to disable the router fuction to use it as a modem only. it has an external ip address as any other modem, but asigns an internl ip address to the network.

How can i make computers connected to a router communicate with others computers that are connected to a different routers and with a different ip address rangein other words:

(computer1)(192.168.1.6)----->|modem/router/switch all in one|(192.168.1.XXX Range)------->|Router B|(192.168.0.xxx Range)------->(Computer B)(192.168.0.5)

I try to set both routers in the same ip range, but that cause problems. I wanna be able to connect to the vpn portion of the routers too.

Thank you, Please help me, for my friend really need me to help him with this.

You don't need the second router. The VPN goes through the router from the ISP.

it is basically a hardware vpn server, and not a software vpn server. It is supposed to be able to receive 254 vpn sessions. I know I can use windows vpn server or even openvpn; however, I don;t want to have a pc on all the time for that purpose. That is wy i got the vpn server, which is a router and a switch too