If you're having trouble sideloading, you can switch to the Meta App Lab version by emailing support@heresphere.com from the email you used to purchase the app from itch.io. This version will autoupdate, and it'll be shown in your main app menu in your headset, so you don't have to sideload.
If you want to sideload the app without using a computer, then I'd recommend using the SideQuest easy installer from here: https://sidequestvr.com/setup-howto
Follow the instructions, and it'll install the SideQuest app on your headset. Go to your apps menu in your headset, switch to unknown sources, launch SideQuest, search for "HereSphere", click on the first thumbnail, it should have an itch.io link, scroll all the way down the page, click on a forum thread link, scroll down to the bottom of the page and click on "Login to reply", login to your itch.io account, click on the HereSphere VR Video Player title link near the top of the page to go back to the HereSphere itch.io page, and then there should be a download button shown near the top since you're now logged into your itch.io account. Click on one of the download links for the version you want to install.