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.
Are you running the latest firmware? If you are and the option is not there then using your DSL-AC88U is out.
Can you put your DSL-AC88U in modem mode and get a router like RT-AC68U to do the per device limiting, though the RT-AC68U wifi is slower than your DSL-AC88U.
If your using a VPN the RT-AC68U can only muster around 25 Mbps to 40 Mbps throughput.
I think it would be best if you make a new spec me thread for a modem/router combo or just a router if you can put your DSL-AC88U in modem mode.
I am running the latest firmware indeed. It makes me laugh that router which costs so much money does not have all this inside already and more purchase is required. I will return this to vendor immediately. should have gone through product description properly. Thanks for all the tips guys, as always, appreciated.