Suggest you look at the full, purchase version of Mocha TN3270 in the Mac App Store. Works on Catalina. The free lite version is very restricted in what it will let you do, and has a ten minute session limit. Here is a link to the vendor's product site. Last updated 3 months ago, so continuing to get developer activity.
Brown University's TN3270 solution was heavily written in 32-bit Carbon frameworks, and Apple has no 64-bit versions of these, so it will not be rewritten for 64-bit compatibility in Catalina.