Installed on Windows 10 Pro (latest) via the downloaded 300+Mb GOG installer.
Game works fine. BUT, cannot install several mod files as there is NO association for file umod, and so far I have tried a few ideas listed here in prior posts to no avail.
Seems like I cannot even figure out how to add a detailed file association in Win 10 either. Its turning into one of those days.
Can anyone help me with this? As I really would like to get these mods added in.

Thanks.

You should be able to apply the registry keys to manually associate umods with UT.
Someone else has posted something on this previously, see if it will work for you.

https://www.gog.com/forum/unreal_series/how_to_install_umods_unreal_tournament_99_mods_ex_chaosut

There had been a program in the past called Unreal Services // UMOD Tool. AFAIR it can do all the registry tricks automatically and even provide some additional options for installation and uninstallation of .umod files.