SIP works, IAX2 not (FreePBX to Zoiper over OpenVPN)

FreePBX 12.0.74,
Asterisk (Ver. 11.19.0),

I used (parts of) this guide to get Zoiper to work over ‘the Internet’ / OpenVPN tunnel connection; (ommited the parts, where they advise about the Digium card, cause I don’t have one)

using ngrep -d [the openvpn interface] port 4569, i can see some hex data being exchanged between the PBX either way but then it seems to stop and Zoiper says registration timed out;
Nothing shows in ‘full’ log or in asterisk -vvvvvvvr (not sure exactly how many 'v’s I tried) while the hex data seems to be being exchanged (screen is my friend).

EDIT3: just found, that SIP account works no problem, talks, rings in/out etc.
[edited out what was irrelevant, since SIP works ]

Something’s wrong with my IAX2 settings then, but where ? Because when I make myself a SIP account, with allowed network set to one as in OpenVPN server ‘client network’ setting, it works.

( I just used default proposed IAX2 settings when making the device up)