Amazon.com: Zyxel High Performance 2GbE SPI/500Mbps VPN Firewall with 200 IPSec and 50 SSL VPN, 8 GbE Ports and High Availability (ZyWALL310) : Electronics
networking - How to setup OpenVPN behind firewall / router - Stack Overflow
How to permit l2tp ipsec vpn through Mikrotik firewall - Timigate
Amazon.com: ZyXEL Next Generation VPN Firewall with 1 WAN, 1 SFP, 4 LAN/DMZ Gigabit Ports and 802.11ac/n WiFi [USG20W-VPN] : Electronics
Issues with port forwarding when setting up VPN (my guess) : r/VPN
Configure Port Forwarding and Test VPN Connection on Windows 10 Part-4
networking - Exposing a port on the LAN behind a VPN Client - Super User