@SioCat
My Insignia RokuTV was always disconnected from the network regardless of being set for Fast Start or not. However, you can try enabling it under Settings/System/Power/Fast TV Start. (places TV in more of a standby mode that has helped some users with connection issues.)
https://support.roku.com/article/115004035608
https://image.roku.com/c3VwcG9ydC1B/Roku-TV-User-Guide-14-0-en-US.pdf
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As for replacing the RokuTV, you could consider just purchasing a Roku device and attach it to your RokuTV. Users have done this to "upgrade" their RokuTV when having issues. You use the device for your streaming needs and the RokuTV just for the OTA/antenna features.
However, recent upgrades to the RokuOS seem to rely on an internet connection for the OTA EPG (program information guide) so may still have these annoyances when not connected to the network.
To be frank, I abandoned using my RokuTV altogether due to performance issues and always being disconeected from the network, and instead just use Roku devices on my non-smart TV's. (although they will work with any brand of smart TVs via HDMI if a user wants to use a smart tv).
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, 4802, 4850, Premiere+ 3921, Express 4k+ 3941, Streambar 9102