How to merge networks on Mikrotik and ISA2006 Server or Mirkotik-Raspberry Pi+3G?

Question

VPN

there is a home network with white IP and ISA Server 2006 as a firewall and ruter.
There is a MikroTik RB951Ui-2HnD with a 3G or 4G modem. He will distribute the network to devices in the country.
An IP camera is also connected to it.
The task is to get access to the IP camera in the country house and smart home devices that are being developed there from home.
Given that you can’t get a real IP on 3G and 4G at all, then most likely this is done by establishing a VPN channel from mirkotik to ISA Server 2006. There are no problems with this, the problem is that ISA correctly routes packets, directing them to the incoming VPN tunnel to cottage network.
How to do this I have no idea.
The second option is to put Mikrotik at home as a router, configure ISA in the back 2 back firewall mode, and put Raspberry with a 3G modem in the country and raise a VPN tunnel from it.
I'm not an expert in configuring networks under Linux, let alone routing under Linux. Buying a second Mikrotik at current prices is not possible.

Answer 1

You set up a VPN server on ISA (PPTP is simpler, L2TP over IPsec is more complicated), configure mikrotik as a client, and then traffic routing - the ISA server has a "network-to-network VPN" connection, so work through it.
wiki.mikrotik.com/wiki/Manual:IP/IPsec#Site_to_Sit...
wiki.mikrotik.com/wiki/L2TP_%2B_IPSEC_between_2_Mi...