How to increase the quality ?

First time I use Plex outside and remotely but I realised the quality not top notch although the file and my server is high-end and it can easily transcode 4k etc . Any idea why and how to increase it ?

First time I use Plex outside and remotely but I realised the quality not top notch although the file and my server is high-end and it can easily transcode 4k etc . Any idea why and how to increase it ?