If it "still works great with your antenna" it sounds like you need to rerun Guided Setup on the TiVo to get the info for the new provider. You cant "hot swap" between the two.
Make sure the card is activated!
TiVo Premiere XL4 (2 of 'em!)
TiVo Mini (Codename: UNICORN)
Retired: TiVo Series 2 ST, Philips Series 1 HDR212