I believe I have both the mk2 bootloader and modem. I've tried switching to different network types and apns, with no luck. I'm not sure about the 700 band thing. I'm considering just selling my i545 on ebay and using the money to buy the tmobile model or international model.
If you're on MK2 modem and Bootloader you should be able to stick a Tmobile sim in there and switch the phone to Global mode. That should get you LTE. I've done it with my sister's totally stock i545. I didn't even need to mess with APN