Could we see the path you supplied for your icons?
What happens when you do NOT use the app on your mobile device, and use the browser instead?
What happens when you use the browser on a different workstation?
The easy-to-overlook mistake which I’ve seen several times is a hard-coded localhost as part of an image or icon path, which won’t resolve anywhere away from the development workstation.
When using the IOS browser it does the same thing, no icons. The machine I’m using which it works on is not the server, that’s cloud hosted but it is the machine I use for the designer.