I think I know which is the problem...
I have flashed stock ROM > root > Debranding + Unlock.
I bought 2 pre-paid SIMs for 2 others carriers (CLARO and MOVISTAR) plus my current contractual one (ANTEL).
None of the 3 SIMs have worked in any of its 3 bands: GSM, HSDPA nor LTE.
LTE is OK that it doesn't work since the bands 2 and 4 (used by the 3 carriers) are not supported on the SM-T807P: https://www.gsmarena.com/samsung_galaxy_tab_s_10_5_lte-6235.php
Then I can't use LTE with my local carriers. That's OK, no problem.
Well, when I'm in IOTHiddenMenu > ##DATA# Menu > HDR/1X Selection, I have only this options:
2) CDMA Only
3) EVDO Only
4) EHRDP 1x
6) LTE + CDMA + EVDO
7) LTE only
Then, I can see that there is not WCDMA support on my SM-T807P!
And now I can understand why is not possible to set and save WCDMA by:
UE Setting & Info
WCDMA Band Preference
**RADIO SETTINGS** > Set preferred network type
You can touch the WCDMA related options but the option is not being set.
It is being set only if it is a LTE or CDMA option related.
My SM-T807P is only taking LTE & CDMA , not WCDMA.
I don't why but I will be researching about if it is a restriction on the SPRINT ROM.
I have not answers for this behavior...