No, the credentials are likely to be correct or else the CHAP authentication would not succeed…
2014-07-08T13:04:04.947803+02:00 linux-z7bd pppd[2333]: Connect: ppp0 <--> /dev/ttyUSB3
2014-07-08T13:04:04.973545+02:00 linux-z7bd pppd[2333]: CHAP authentication succeeded
2014-07-08T13:04:04.978840+02:00 linux-z7bd pppd[2333]: CHAP authentication succeeded
2014-07-08T13:04:25.307054+02:00 linux-z7bd pppd[2333]: Terminating on signal 15
2014-07-08T13:04:25.310037+02:00 linux-z7bd ModemManager[545]: <info> Modem
/org/freedesktop/ModemManager1/Modem/3: state changed (connected -> disconnecting)
2014-07-08T13:04:25.318784+02:00 linux-z7bd pppd[2333]: Connection terminated.
Or still too bad UMTS connection in my village? At least it gets that far. I tried multiple times. It always ends that way.
It may be the case that the signal level is insufficient. For marginal areas, a 3G router with external antenna may perform better. If practical, try testing nearer the cell tower, or wherever the signal is known to be stronger. Using a mobile phone can be helpful to determining the signal strength at a given location.