No. The Twonky DLNA media server is embedded within the My Cloud firmware.
Generally when one enabled the Twonky Media Server through the My Cloud Dashboard all devices on the local network that are capable of streaming DLNA media will be able to access the Twonky Media Server on the My Cloud to stream media.
One can use the My Cloud Dashboard to block certain DLNA clients using the View Media Players link.
Yes provided the SRT subtitle file is named EXACTLY the same as the movie file. Or one can embed the subtitles into MKV files. This assumes of course the DLNA client supports playing subtitles.
If you haven’t done so already, download and read the My Cloud User Manual which explains the My Cloud features and how to use them.
https://support.wdc.com/product.aspx?ID=904
Also see the following unofficial FAQ that has additional information/suggestions when it comes to the Twonky Media Server that is embedded within the single bay/single drive My Cloud units.
https://community.wd.com/t/faq-twonky-dlna-media-server-setup-use/95373
