Try going to the play store on your computer and searching for the Samsung browser. When you find it, click install and if you have more than one phone, select your Note 5 in the list and it should install. This is how I got it installed on mine.
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I see what you are saying. And as you have found you apparently have to download it via your PC. My experience on my Verizon Note 5 was that I couldn't download and use AdBlock Fast before my phone was upgraded to MM even with an apk sideload.