In the past, we used ES File Explorer to sideload apps this way, 
but it now requires a paid subscription to do the job. 
So we’ll use MiXplorer instead

https://bit.ly/3GPH28p 

Click on the three dots at the top right of your screen

Click Servers

Click Start HTTP/WebDav Server

Leave your FireStick on this screen. You’ll need to e
nter the URL, as indicated in the screenshot below, on your computer

it'll be http://192.168.1. XXX:8181 etc
Open a new tab in your web browser and enter the link from your FireStick. In my case,
it’s http://192.168.18.168:8181, but make sure you use your own
on pc 

You should now see a list of all the FireStick folders on your computer. 
Click the Downloader folder, which is where we will save the APK file

Click Choose file

Browse to the location where you downloaded the APK file in the first step. 
Then, select the APK file and Open it

Click on Upload

Install APK Files Transferred From PC to FireStick

on your FireStick

Click Stop HTTP/WebDav Server

Click on the X button

use downloader to install from downloader folder in app