My phone is rooted and I manage to lock it on 4G using the app mentioned above. I have a video demonstration. I just don't have time at the moment to upload it on Youtube and share. But I will do so before 9pm today.
No need to use Telkom for calls because, most calls are made through Safcom. So a dual sim phone is good to go.
When the phone is locked to 4G it does not fall back to 3G/2G. Luckly, thanks to Telkom's poor 4G coverage.
The video is simple, once I use the app to lock phone on 4G, the phone switches to emergency mode when no 4G signal can be picked, which is a clear indicator that it is locked on 4G.