Buffering

Hi

advice needed. I share access to my server with family members and they all have no problems streaming my content.
however I have one member who has constant buffering issues. I have tried all kinds of devices - xbox, shield box, android box etc… and all buffer.
he has a great fibre internet connection and all other streaming apps work great, only plex seems to suffer.
he isp is talk talk - and I am wondering could the problem be some restriction in the router settings ??
I have tried everything I know - with the exception of the router
any help would be great ?
thks
gp

Mod Edit: Please don’t double post… // closing this post.