In the same direction, I once wanted to test an embedded device on crap wifi.
So I just ordered the cheapest AP I could find.
Except the damn device worked perfectly. Slow but rock solid.
One of our testers at $CURRENT_JOB also has trouble simulating a crap network, because our network is good.
Some proxies, iptables extensions, and OS-provided tools exist - there's almost certainly a combo that would work for them. What platform?
Unless it's for a custom physical device, then uh. idk. Probably something, proxying through another computer that is hosting a separate wifi network? But likely a lot harder.
You can simulate bad wifi with the throttling option on the network tab of your browser's developer tools