I can't remember if the zyxel uses xdslcmd or xdslctl.
For my ZyXEL VMG1312-B10A it's:
adsl configure --maxDataRate ***** **** 100000DS desired rate followed by US desired rate (or max.) in Kbps as j0hn has said.
In my case entered from command line after logging into modem via Telnet, Win 10 OS.
I believe application of a modem cap instigates a re-synch, have never tried.
The cap won't survive a modem re-boot, if still required will have to be re-applied.