Remote Access connects for about 15 seconds then disconnects

Hello, I am writing from Barcelona (Spain). I have been using Plex for a few months to watch series and movies on my Smart TV. So far everything perfect. Activate remote access and opened port 32400 on my router (Movistar Fibra Óptica), and invited 2 users to access my content. Everything worked fine the first day and then (yesterday) remote access stopped working and I can’t find the solution.

I have iMac. Could someone please help me find the solution? Thank you.