Downloading from unidentified developer

A trusted developer has provided me with an app that allows me to open an older scanning program that I need. TrendMicro won't allow the dmg, saying it "can't be opened because it is from an unidentified developer." Short of disconnecting TrendMicro so I can do this, what do you suggest? It is important to note that I use OS X El Capitan, and not Mohave, because my system is too old to allow downloading a system update. This is why I must have the developer's older version of the application.