Reading through the manual, and looking at the app, it seems there’s no way to toggle an input between mic and line. You just set the gain really high on a mic channel.
The XLR inputs have a maximum input level of +17dBu, so they do accept line level signal, but it will come in slightly hot if at a +4dBu nominal level.
The Combi inputs however, include a built-in 20dB pad on the balanced jack connection, so these (or the dedicated stereo line level jack inputs) are generally first choice for connecting line level signal.
All of the documentation on the CQ-18T shows that all 16 inputs are Mic/Line inputs but if I put a line level source into inputs 1-8 they are WAY too hot, I have to run the gain all the way down and it still runs hotter than any of the mic inputs using the auto gain feature so it doesn’t mix properly. Would it be possible to get a mic/line toggle added via firmware to correct this problem? I think it’s rather misleading to call inputs 1-16 Mic/Line if they can barely handle line inputs on the first 8 (at the very least, something in the documentation should specify that the second set of 8 are better options for Line inputs).