UR824 line output signal strength? Why won't it trigger a compressor?

I am trying unsuccessfully to route Mix2 outputs (channels 3 & 4) to an outboard compressor (BlackLion B172A). The compressor needs a line-level signal to trigger any compression. UR824 “says” it outputs at line level but I have to crank everything to completely unreasonable levels in order to deliver a signal the B172A can see as worthy of compression.

I’m just a humble guitarist, not a pro sound engineer or Berkley graduate.

Any suggestions?