I was (theoretically) able to unlock Band 20 with QualcommTool.exe according to the Info on xda-developers.
First put phone in Diag mode: In the shell input as Admin:
adb devices
adb shell
su
setprop sys.usb.config diag,adb
then
-> run QualcommTool.exe as Admin
-> with right mouseclick first activate an then connect the diag-com port
-> click the tab "bands" then the subtab "LTE"
-> (1) "Read" with activated "LTE BC Config #6828" -the one without "DIV" (I had to do it twice till it worked)
-> then click the selected band you want to add
-> (2) write
-> click LTE BC Config DIV #6829
-> (3) read
-> then click the selected band you want to add
-> (4) write
goto (1) and repeat till (4)
about 3 times, because the settings were not saved, when done only once or twice.
The last steps were in my cases step (1) and (2).
When i read "LTE BC Config #6828" the 3rd time, it was saved.
Then reboot.
LTE Bands are after checking with QualcommTool saved, but i wasn't in a Band20 area till now, to check it practically.
Maybe some of you can test it.