I have a similar problem with a dual-core Ampe A10 tablet purchased from Spemall.
I have installed PPP widget to manage my 3G connection, but I get the "No modem found" error message.
Now everything works fine with my tablet: 3G, GPS.. everything.
I actually haven't done too much to fix this. I just restarted the tablet after the SIM-card wasn't detected, went in Setup, Other Mobile services, Automatic network detection, then entered the righ parameters from my mobile internet service provider, especially MCC (216) and MNC (30). Then, everything started to work, despite PPP widget showing no connection. After all, I uninstalled PPP widget, because I don't need it.
I didn't modified any IMEI number, don't even know how it looks like.
Edited by Mazarin07, 07 December 2012 - 07:02 PM.