1. Countermeasures

# Frontend server with Next.js
next dev

# Backend server with FastAPI and uvicorn
uvicorn app:app --reload

# Frontend server with Next.js
next dev -H 192.168.11.2

# Backend server with FastAPI and uvicorn
uvicorn app:app --reload --host 192.168.11.2

It's necessary to explicitly specify the IP address for both.

2. Background

Trying to check the screen from a smartphone by connecting to the development server 192.168.11.2
The images stored on the backend server are not displayed
Change the URL of the backend server accessed by the Next.js frontend server from localhost to 192.168.11.2
An error occurs with fetch

Not very clear.