Network request fails with fetch in Expo React Native - react-native

Question

I am having problems with a simple fetch request in my expo app. Long story short, the user needs to input a prefix and a port. Then we call a fetch request from the address that is made up of these information, expecting a json to verify whether the prefix and the port are correct.
We keep getting the "Network request failed" error. The full error:
Network request failed
at node_modules\whatwg-fetch\dist\fetch.umd.js:null in setTimeout$argument_0
at node_modules\react-native\Libraries\Core\Timers\JSTimers.js:null in _allocateCallback$argument_0
at node_modules\react-native\Libraries\Core\Timers\JSTimers.js:null in _callTimer
at node_modules\react-native\Libraries\Core\Timers\JSTimers.js:null in callTimers
at node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:null in __callFunction
at node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:null in __guard$argument_0
at node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:null in __guard
at node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:null in callFunctionReturnFlushedQueue
After several days of trying various solutions we found (specifying Content-Type, multiple cors-related options, some tests and changes on the server from which we are expecting the said json, creating requests with axios or XMLHttpRequest) we ended with this code:
fetch(`https://${prefix}.<the rest of the address>:${port}`, {
method: 'GET',
mode: 'no-cors',
credentials: 'omit',
headers: {
'Content-Type': 'application/json',
'Access-Control-Allow-Origin': '*',
},
})
.then((response) => {
// the address we want to connect to works
...
})
.catch((error) => {
// the prefix or port are incorrect
setError('Incorrect prefix or port');
});
We suspect it might be something to do with ports, since request from here https://dummyjson.com/products/1 works perfectly fine.
Another interesting thig has happened though, that would point more at some caching problems perhaps. Today the request worked for a few minutes on two separate occasions, both were after we changed the Content-Type a few times (one of them was when we set it to "text/xml" and then it worked when we changed to anything, like "application/json", "text/html", even without it). We have cleared the cache, but the same thing happened - we changed the Content-Type a few times and it worked, then it stopped.
We created the project using npx create-expo-app AwesomeProject ("expo": "~45.0.0", "react-native": "0.68.2", from package.json), are running it on Android (physical device) from Win 10. We are also using the Expo Go app while developing our app.
Calling this request in the browser works on both PC and the Android phone.
Thank very much you in advance for your time and any help you could provide