Hi Denis
I had a similiar issue on my last NethServer (Installed last month…).
The server is hosted in Germany at Hetzner.
Hetzner provides a Dedicated Server, we installed Proxmox on top of Debian, then installed OPNsense (Main firewall) with two seperate LAN connections (10.49.11.0/24 and 172.26.49.0/24).
My NethServer is on the 172.26.49.0 network.
I can reach this network with normal OpenVPN from OPNsense (Port 1194).
I can also connect using a Site2Site OpenVPN on the OPNsense (Port 1195).
I wanted a third OpenVPN network for direct and exclusive access to the 172.26.49.0 network, I set this up on NethServer. (Port 1198)
AFAIK, NethServer can do this also (several OpenVPNs with different ports), however this takes some manual configuring, as the GUI can only handle one standing OpenVPN configuration.
OPNsense can handle as much OpenVPNs as you need, or have available ports for…
But as I’m here on the NethServer forum, I also wanted one on NethServer. 
Configuring multiple Subnets is also a challenge for the OpenVPN GUI in NethServer.
Maybe the free version of Pritunl could cover you with a quick & comfortable install - I’m thinking of using that for larger installations…
My 2 cents
Andy
This is my Home Setup (OPNsense):
One OpenVPN for Roadwarriors, the other is for Site2Site connections.
Note:
I often use a parallel IPsec V2 based VPN for Site2Site, only not all sites can use IPsec. For Example, in Germany, as it’s hard to use bridging for the routers, almost always uses OpenVPN except for larger companies / Institutions…
OPNsense is quite different from NethServer, also it’s based on hardened BSD. But it might still help to see how ports and networks are used…
Security:
DH. 4096
Encryption: AES-256-GCM
Auth/Digest: SHA512
Strict adherence to certs configured!