Regardless of the configurations that need to be done on the RT-AX88U, you need to rework the VLAN configuration on your RB5009. You are currently not following the best practice, which is to use Bridge VLAN Filtering with only one bridge, and to put the VLAN interfaces under that bridge:
See also http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/131 You are wasting the hardware offloading features of the RB5009 with your current configuration.