hi Guy I used the site Softether.org
and the best is to go for a 16 GB flashcard for the VPNserver and Raspi3 or Raspi4
The core and Pi3 do not need to be on the same subnet - here I have two VLANs and the main thing is ROON core only advertise on ONE net not multi - SO be sure the subnets all are accessible both sides v.v. if not you will see a Connection to the core but NO data …is popping up in the client. Indeed you need to forward several ports for ROON to the VPN server … these are UPD 500, 1701, 4500 TCP/UDP 9100-9200 and TCP 1723
Mind installation of VPN server is pretty straight forward, and also ports not to difficult, but Make sure you set ups IP/SEC L2TP otherwise NO work 
For iPhone I use the regular VPN IPSEC
hope it helps a little… regards,
Max