In network mode option we will not be able to find a selection of 3G/HSDPA only or for 4G/LTE only in OPPO smartphone series regardless. This may be done by manufacturing companies in order not to complicate the layman user. So, the choice of LTE set automatically in the default setting.
Fortunately the Oppo doesn't modify the full version of android that is in use on the device. We can still find LTE settings only and WCDMA or HSDPA only through ways below:
2. Run the application, and then select the second menu. it is the "Android Testing"
3. Select Phone Information 1, for SIM 1 and Phone Information 2 for SIM 2 . Choose the SIM which you are using for the data connection to the internet.
4. Choose LTE only or WCDMA only in part Set Preferred Network.