No need to bother with messing around with ssh into box, just use File Manager but first ensure you are configured in KODI Settings so you can see and write/delete to hidden and system files. In KODI 17 default Estuary skin go to Settings/Media/General and enable the last two items which are disabled by default, cannot right now remember where you find thsoe settigns in the old Confluence skin which I no longer use. I prefer Estuary skin now at last I am familiar with it.
So with the font TTF file you want to use (I use Arialuni.ttf file by the way) on say a USB stick or SD card and in your box run up File Manager. In the A window find your font file on your memory stick. Next in the B window go to this path root/storage/.kodi/media and once there make a new folder called Fonts. Open your new Fonts folder and copy across your new TTF font fiile from your memory stick. Reboot and that is it. You can now select your new font in the subtitle settings.
I create a new source in File Manager called root which points to the the Root File System and the path just shows the / character. This is so useful as I can access all the folders in /root and then using File Manager copying across config files like remote.conf, font files etc and of course new LE update files into the storage/update folder. This is by far easier than running up Putty or whatever on your PC, getting your box's LAN IP address and having to log in via SSH trying to remember the various commands syntax.
Just put what you need to copy across onto say a USB stick. Or do what I do and create another File Manager source pointing directly to my NAS Drive (or any other local network storage location), where I have accessible folders with all of these needed KODI files stored and can easily find them and copy across what is needed simply using File Manage every time. I actually only use SSH for editing, or makingfrom scratch, a remote.conf file by finding all the required remote's key codes, plus of course for installing LE from SD card to internal, which I always do as see no point in keepign bloatware Android with LE being so much better, leaner and so much faster running properly from internal.
Hope that helps.