It is not legal to run an AC condensate drain into a sewer. The only
thing that should tie into the sewer is plumbing fixture drain. The
drain of the unit should go outside. Cut it apart in the attic and use
a shop vac to suck out the main drain pan in the indoor unit. Then
suck the drain line that disappears into the insulation. With the AC
OFF so it doesn't make water for a while, go outside while the vacuum
is running and listen for the sucking noise. You will find where the
drain line comes outside. In the future you will be able to vacuum it
from there. It is usually next to the outdoor unit here, but sometimes
is put behind bushes to hide that ugly thing. :-)

You should not be working off the emergency drain pan. You can run a
drain line from the emergency pan to the soffit so you will know the
next time you have a problem before the AC shuts off or the ceiling
comes down.

Stretch