I'm maintaining a web site and was using ATC for transfers with my old Amiga, but there seems to be some incompatibility with MOS and ATC, can upload only 1 file at time, downloading doesn't work at all, so I'm open to suggestions for an easy-to-use FTP client (GUI is a must!) for MOS. So far tried SimpleFtp and GUI-Ftp from Aminet, do not work.
Try the 2001 version of ATC. Try OpusFTP (built into Magellen). Try FTPMount (aminet, a bit annoying to set up, but it will treat your ftp account like a drive!)
I downloaded the xx.Dec.2001 version of ATC and it works fine with MOS. The latest does NOT (at least here it freezes if trying to upload multiple files or download anything).