I am afraid you are doomed.
E-Plus (together with O2) is well known to not care about such specific customer problems. Even more as the N9 is really exclusive (at least for them, maybe they even do not know this device at all ).
And we cannot help you as it is a problem of network/operator with this device.

One possible workaround (instead of reboot):
enable developer-mode, open x-terminal, type
devel-su (followed by the root password: rootme)
and type
/sbin/initctl restart xsession/icd2

Maybe it helps, maybe not.
If yes the thread about ProfileMatic is worth to read, allowing you to automate this maybe.


--
another idea/question (just a shot in the dark, as I do not use this service):
you wrote it works most often one time. Does it work more often sometimes? Were you connected already to 2G/3G network? Maybe internet and/or MMS settings do not like each other?