Huawei P8 max Quick Manual page 7

Dual SIM settings
SIM card 1
2G/3G/4G
SIM card 2
2G
4G/3G slot
默认移动数据
默认电话/信息
SIM card SIM card
1 2
When you insert two SIM cards, the lower card slot is set as the primary card
slot by default and supports 2G, 3G and 4G networks. The upper card slot is
set as the secondary card slot and supports 2G networks.
To switch the primary and secondary card slots, touch
and set Card 2 as the 4G/3G card slot.
management
If you only insert one SIM card, the card slot with the inserted SIM will
be set as the primary card slot.
Settings > Dual card
3