Solid file explorer. Link Google drive account.
Place APK in your Google drive.
Do you have an Android phone? If so try the send files to tv app. You'll download it to your phone and tv. Can send pretty much any file to it this way. I found it much easier than using downloader.
stop trying to use downloader and download send file to tv on your TV and your phone then open the app on both your phone and tv your phone is the send and your TV is receive after you send your app you are going to need a file Explorer to install the apps on your TV. this is much easier then downloader.
I use adb from a computer, or use easy fire tools from the play store.
Just get an FTP app.
open the ftp address from your phone or pc and put the apks that way
You can use this now to install apks, files etc...!
Try aptoide tv
I would suggest installing apk through usb drive. Android tv sideloading apps are not reliable nowadays