How to connect two networks over the Internet?

Question

VPN

Good day!
On the one hand, there is an ASUS RT-87U network - 192.168.1.* /255.255.255.0
On the other hand, the Kenetic GIGA 2 network is 192.168.2.* /255.255.255.0 with a plugged-in 4G modem (192.168.8.1)
Tell me how to properly configure connections so that everything works correctly in both directions. Write if possible, please!

Answer 1

.. or simply - raise VPN

Answer 2

VPN servers are up on ASUS RT-87U. I connect with Giga2. How to make a return?!