Network Link Conditioner should do what you want.
It's part of "Additional Tools for Xcode" and can be downloaded here:
Attach your iPhone to your Mac, launch Xcode and go to...
Window > Devices & Simulators
Select your iPhone in the sidebar (unlocked and displayed without errors)
On the iPhone you should now have the Developer settings:
Settings > Developer > Networking > Network Link Conditioner
See also: https://nshipster.com/network-link-conditioner/#enabling-network-link-conditioner-on-ios-devices