Plex is cut with synology ds216 + II

I have a synology ds216 + II that transcodes 1080 and when I try to see something in plex using a smartv lg or in iphone or ipad, in local network 5ghz with a router asus rt68u (discarded router problems) the video is intercalated and after a while a Message "the server is not powerful enough to convert this video and to play this video." I've tried changing the plex configurator on the server but it stays the same. Do you know what the problem may be?
The support of synology is disregarded because the plex application is not theirs.
Thank you.