Well, if it's the same problem that I had, it's because your APC UPS does not support the "smart" signalling protocol.
APC uses two types of signalling: "smart" which is RS232 based and passes lots of data in ASCII strings about the status of the UPS and this is what RouterOS supports. What I like to call "dumb" signalling just sends a couple of lines on the interface high and low to indicate "on battery", and "battery low".
It is difficult to know which APC UPSs support smart signalling but anything with "Pro" in it seems to have it. The "Back UPS" range does not. Check with APC tech support before you buy. If you have already bought a dumb model joint the
Hope this helps.