Having a remote OpenVPN server at your home and connect to it with the RasPi is a good alternative if you cannot use port forwarding on the modem. But there are some things I do not understand so I will give a drawing to get it clearer and that we can correct to your setup. As far as I understand you would like to have this setup:
10.10.10.2 ┌──────────┐ 10.10.10.1
/ vpn-tunnel │ │ \
(tun0) ═════════╗ ╔══════════════════════════ VPN-SERVER
station <.~.~.~.> (wlan0)RPI(ppp0) <-----> MODEM <---> │ INTERNET │
\ wifi / \ usb wan │ │
(dhcp 192.168.50.1 (dhcp └──────────┘
from RPI) from MODEM)
Is it this what you want?
(Will be continued)