Correct, in such case you would need to have a separate device, which would be configured as VPN Server. This device could be another Teltonika router, your home computer, office server, etc. Just take note, that this device must have Public IP address, so that your RUT955 (as VPN client) would be able to connect to the server.
Configuring VPN is easy, you just need some practice. To begin with, decide which VPN protocol you would like to use:
- OpenVPN is most versatile one, but it would require you to create your own certificates, which takes some time (All necessary information about OpenVPN, including creating the certificates and troubleshooting can be found here)
- GRE tunnel is not suitable for you, since it requires two Public addresses (on both tunnel ends)
- IPsec, PPTP and L2TP is the quickest ones to configure, so you can try using one of these protocols (You can find information how to establish these VPN tunnels here: IPsec, PPTP, L2TP)
Then follow appropriate guidance and you should be able to establish VPN tunnel in no time.