For those of you out there having connection issues even though you have full bars on your new Samsung Galaxy S2 HD LTE phones or any LTE phones for that matter here is a solution I got from a bell manager. The reason why you get "Mobile Network Not Available" is because most Cell phone providers are having issues with the LTE network routing tables. To fix the issue you have to go into your wireless settings-> Mobile Network settings->Network Mode and change the mode from GSM/HSPA/LTE (Auto Mode) to HSPA only. Once that has been changed you should no longer have the connection issues. Before this fix I was missing phone calls left and right as well as getting frustrated with rebooting my phone every time it would conk out on me. Please note that this will keep you from getting LTE speeds, check back with your Cell Phone company on when the issue will be resolved so like that you can change back to LTE.
I hope this helps others out there since apparently there is a master ticket going around bell that even their reps don't even know about. Always ask for a manager for this issue thats how I found it.