Create a NetworkManager connection for NIC

Hi All. I use SoftEtherVPN. It comes with a client program which I have configured. It creates a nic for the L2 VPN, and I think I now need to add it to NetworkManager so that it becomes usable with an IP address. I see the device called

[root@gabriel network-scripts]# nmcli device show vpn_softether
GENERAL.DEVICE:                         vpn_softether
GENERAL.TYPE:                           tun
GENERAL.HWADDR:                         5E:15:70:48:4B:97
GENERAL.MTU:                            1500
GENERAL.STATE:                          10 (unmanaged)
GENERAL.CONNECTION:                     --
GENERAL.CON-PATH:                       --
IP4.GATEWAY:                            --
IP6.ADDRESS[1]:                         fe80::5c15:70ff:fe48:4b97/64
IP6.GATEWAY:                            --
IP6.ROUTE[1]:                           dst = fe80::/64, nh = ::, mt = 256

I have tried setting up a new connection with network manager, but when I attempt to bring the connection up I get

Error: Connection activation failed: Failed to find a compatible device for this connection

Can anyone please advise how to add an IP address and bring the device online?
Many thanks

How, exactly? What command?