It's a long post but nearly covers all my experiments and changes I did to my NM. Hope the information is complete and if there are still question, more information can be provided.
I've a ZTE AC2726 USB Modem (CDMA modem) which worked fine in Ubuntu 10.04 /11.10. I recently switched to 12.04.1 (precise pangolin) All 32-bit. after the switch the first issue I faced was to connect to internet using my USB modem (ISP: Reliance Brand: Netconnect).
Tried to run the drivers provided by Reliance but they are old and won't support Kernel 2.6.30 above. since the code was not downloaded with ISO image (of 12.04) i couldn't compile the files provided in such driver.
lsusb does detect it as Modem with output similar to 19d2:fff1 ZTE CDMA technologies inc. (or similar as i didn't note it down) If it is detected as USB storage it shows 19d2:fff5 (as per few online forums, i may be wrong here).
I used the network Manager and configured the modem to dial #777 (default) and the ISP provided username:password combination. It tries to connect to internet (3-4 times automatically)but fails to get online.
once I was able to connect in the monring hours and the message was flashed 'registered on CDMA home network'. I was able to run an update. the kernel was updated with 3.0.2 -pae OR something similar (can find out if required). I surfed the net for about 2 hours later before restarting.
After the restart, again the Modem was not able to get me online. I kept trying for many times. I tried changing the setting in NM. One evening after dark I was able to connect to network with same message flash 'registered on CDMA home network' (the message was similar, i'm not precise here,sorry). I was able to surf the net for nearly 3 hours before I switched off my Laptop.
I'm not able to get online after that day, It's been 3 days now. I'll try the observered theory of late/early hours sometime soon as mentioned below.
Laptop configuration : Make: Lenovo Model: B480 Processor: Intel B950 RAM: 4G DDR3 HDD: 500 G Broadcom Wireless/Bluetooth/Ethernet LAN
OS: FreeDOS / Ubuntu 12.04 LTS (dualboot) - 32 bit Kernel 3.0.2-pae -32-bit
ISP: Reliance Communication (India) USB Modem: ZTE AC2726 (Black/silver)
Obeservation : I'm able to connect to internet in those hours when generally the speed is high (low usage by other network (wireless) users). like in early mornings or late nights. This is strange as connection should not be dependent on bandwidth usage.
Any help would be appraciated to fix this issue. before I decide to cahnge the OS or ISP.