After some research, my understanding is that PowerApps is actually acting in the middle here - users are not directly sending requests to api.ipify.org so even if all works and it still won't return the user's IP.
PowerApps needs to have some built-in way for us to get user IP, as well as their network/device information. Using a custom connector is not feasible.
Thanks for posting this to the new idea discussion board, @henrypei! I will go through the General Discussion Forum and try to find users running into the same issue as you so that they can upvote your idea.