The problem probably lies the Samsung Smart TV’s DLNA client. Seems the Samsung TV’s have a lot of problems with the My Cloud based on many past threads involving Samsung SmartTV and the My Cloud. If you do a forum search (magnifying glass icon upper right) and use the search term “samsung srt” you’ll find several previous threads detailing others having the same problem with SRT files on Samsung TV’s.
I have no problems playing subtitle SRT files with a Roku that streams from a My Cloud using DLNA (Twonky Media Server).
You may want to review the following thead to see if anything in there will help.
https://community.wd.com/t/faq-twonky-dlna-media-server-setup-use/95373
