Maybe the first thing to check is whether you have a powered on command control setup (TMCC or Legacy) anywhere in the vicinity. If so the M-C will probably default to TMCC operation and not run in conventional.
Someone with more tech knowledge than me may answer - I have only installed M-Cs in a command control setup, where they have worked reliably (when I didn’t make a mistake in into stall action). The manuals for these only refer to conventional operation in one place, and refer to a built-in reversing unit. All the other instructions are for TMCC. Also they won’t drive RailSounds other than in older systems, not ERR’s later versions which are the only ones available now.