I know you can use your phone as a touch pad/ mouse when in dex mode on the phone. But I have a tab s7+ which I constantly use dex on and spare note 9 that I rarely use and would be great if I could use it as a mouse when my tab is in Dex mode.
There was a really good app on the play store that works like this... but I forgot it 😑. Try looking on YouTube channels TechWiser/mobiscrub/Beebom and search your query along with the channel name. Hopefully, this app would be found in one of their top 10 lists, shown in the top 3 search results 😎