Font does not work in Safari

the package you can download from google on fonts that have “variable fonts” contains a subfolder with those static fonts.
Enfold could handle variable fonts – but you had to know how to work with those variable fonts.

if you only like to upload those static fonts – pull this folder form that zip file to your desktop – rename it to “Montserrat” and (important) zip it for PC ( not including the invisible OSX files generates with their compression options.). Betterzip f.e. has a drop-window where you can choose for what system you zip it.
Why this is important. if you upload that zipped file you will have an additional font in there:

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Next : Google does only offer ttf files for you. but it might be better to upload woff2 files ( brotli compression ) as font-files.
( less loading times on woff2 ). So it might be good to have both woff2 ( for modern Browsers ) and ttf (for older browser support)
see: https://kriesi.at/support/topic/host-web-font-yourself-some-info/#post-1364066
use transfonter to generate woff2 and ttf files – put them in one folder – rename the folder to your needs ( Montserrat ) – and upload that.
As mentioned there – think if you realy need an italic font to upload – because browser rendering might do a good job to of normal font.