Network type Edge despite setting to WCDMA Only

[AC Question]

I have a new Android phone - Doogee Valencia 2 Y100 Pro. Terrific value, only I'm having trouble with the data network access. It downgrades to 2g way too much, in fact most of the time, even though I know the 3G coverage is very good where I am. I learned about the testing app, where through entering a service code (*#*#4636#*#*) I can access a hidden menu and select the "preferred network type". I learned that if I set it to WCA Only", it should stay in 3G, and failing coverage give no data access rather than downgrade to 2g. When I do that, service stops temporarily (while restarting the radio I'm guessing), but when it comes back up, it's as if I didn't change that preference at all. After a while, and without restarting the phone, the preference in that menu goes back to the original setting, which is "GSM auto (PRL)".

Essentially, I'm failing to get the use of that preference setting, and most of the time can barely get any data throughput. If you have any ideas, even leads for troubleshooting, it would be great.