I found the solution to be manually editing the APNs in "Settings->Connectivitiy->Mobile Network->APN" and not only selecting my SIM'S APN (the default was some english carrier), but also changing it's type (under 'Settings->APN Type') to "default" (have to type it in manually).
Fixed the problem for me.
# phone/model> HTC One X - EndeavorU
# phone/ROM> ViperX 3.6.1 (Android 4.1.1)
# phone/Recovery> Some CWM variant
# tablet/model> Asus Nexus 7 16GB (WiFi)
# tablet/ROM> PARANOIDANDROID 3.50
# phone/recovery> TWRP 184.108.40.206 Touch Recovery