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From flathub.org — the developer's official site. Flatnic doesn't host, re-package or modify files.

How to install Serial Port Assistant

Click Download — the developer's official website opens in a new tab.
Download the latest version there and run it. Choose Custom/Advanced where offered, and skip any bundled extras.
Prefer the command line? With Flatpak you can install and update it with: flatpak install flathub io.github.KangLin.SerialPortAssistant
Open Serial Port Assistant once it finishes installing.