It's obvious, but... Don't use WDRT on older Lumias - anymore!
(All About Windows Phone)

This is somewhat obvious if you think about it, but it`s worth expanding on anyway, since it almost caught me out yesterday. There are a number of readers with older phones - Lumia 640/XL, 730/735, 830, 930 and 1520 is a good (though not definitive) list - which started out on Windows Phone 8.1 but which were offered Windows 10 Mobile, at least via Microsoft`s Upgrade Advisor utility. And have been running W10M smoothly. But now in 2020, in the event of problems, don`t even think of using long time stalwart utility WDRT (Windows Device Recovery Tool) to restore your phone`s original OS in order to `rebuild`. Let me explain...