@Malakai
i'm sorry, sometimes i forget the tweaks applied during the years.

gprs0 interface has default mtu at 1400 but can transport packets up to 1464 (= real mtu is 1464)

So, to gain a bit of speed (4-5%) you can tweak that value with:

ifconfig gprs0 mtu 1464

and you can check this with:

ping -M do -s 1436 8.8.8.8

(since 1436 + 20 bytes for ip headers + 8 bytes of icmp request = 1464)

Trying 1437 will fail returning no packets transmitted.

Now, about opevpn, it seems that mtu discovery is a little bit messy resulting that the parameter "mtu-disc yes" produces long connection times and some strange behaviors (openvpn developers still suggest to avoid mtu-disc and fix things adjusting by hand the mtu on both sides or using the mssfix option)

So here we are, mssfix should consider the real mtu less the openvpn overhead due the tunnel encapsulation (it changes with udp/tcp)
Openvpn howto suggest to start trying with some values 1200,1300,1400 to discover if it fixes the transmission problems.
In fact, with N900 stock mtu at 1400, mssfix must be near 1300 (i'm sorry at the moment i can't point you to a proper page describing detailed openvpn overhead calculation)

After this confusion and i hope explanation, maybe fw190 has still some other (basic) trouble with maybe a dns failure or a missing proxy setting required by that provider.