If you don’t have admin access to the router, it's pretty hard to limit the Wi-Fi speed for a specific device directly. Most routers require admin login to set per-device bandwidth limits using QoS or similar features.
That said, there are a few workarounds depending on your setup:
Use your PC as a middleman (hotspot or Ethernet sharing):
On Windows, you can use tools like NetLimiter to control bandwidth for devices connected through your PC. On Linux, tools like tc or wondershaper can also help with this.
Custom firmware router (if you can flash one):
If you have a router that supports DD-WRT, OpenWRT, or Tomato, you can install custom firmware and configure QoS to set speed limits per IP or MAC. But again, you'd need access to the router to do this.
Mobile hotspot:
If you're sharing your mobile connection, some Android apps (on rooted devices) like NetGuard or AFWall+ let you manage bandwidth per app or connection.
If you're trying to stop a specific device from hogging all the bandwidth and can't touch the main router, your best option might be to create your own private access point using a second router or a PC acting as a gateway.