Best answer by Mohammed
So we can understand this better, is this for a P20 Lite you've purchased on a contract with us or for one purchased separately and using our SIM in the handset?
It could be a firmware issue due to the device having a different networks firmware, thus not showing the option here. Have you tried updating to the latest version of EMUI?
According to our list here, it's definitely a supported handset.
My understanding would be the issue could be caused by the firmware.