You need to understand what spec is the ISP supplied router. Most router issued by ISP are crap for WiFi and some broadcast a separate SSID for customers nearby such as Xfinity. However, you can disable this WiFi on ISP issued router, hook up your own router (for mesh network and configure it to optimal setting) or hard wire directly to your Plex server so you have a reliable connection. Next, understand what native format supported on the Fire Stick so you know whether transcoding is involve for video container and audio codec. For my ChromeCast, it does not support 4K or MKV and 5.1 audio so it will always transcode your the next available 1080p/720p. I’ve converted all my media to 1080/720p h264 mp4 for universal support and HEVC for 4K so most of my media are using direct play and stream. I’ve notice some issue where casting from the Plex app may transcode/direct stream and cast from Chrome browser will direct play. My library is shared with friends and family so I need it to be in a container and audio format that most TV, box or caster will support so everything can direct play and will be less taxing on my computer that hosting the server.
Whatever client you will use, check the tech spec on video and audio playback if it support your media format. If not, your server may be crippled if it can’t handle transcode or it won’t play at all. I know Samsung TV support MKV natively so you should be able to direct play them.
Bridge mode will not work if you cannot connect your Fire Stick to it. However, if connection is not strong, set it as WiFi repeater mode instead if the router supports it.