This is from a RT-AC68U interface.
Bandwidth limiting per device can be done under the QOS page.
Should be something similar on the DSL-AC88U interface.
Latest firmware for the DSL-AC88U is dated 2018/12/14 and Asus released the DSL-AC88U around 2017 and just after about a year no more firmware update.
Also the limiter function do not work properly if using a vpn.