Network request failed with react-native and graphql

I have a react-native app, that has a GraphQL client to get the content from my server on Heroku.

This is how my GraphQL client on the app looks like:

// API_ENDPOINT: https://my-app.herokuapp.com
const httpLink = new HttpLink({ uri: config.API_ENDPOINT });

const authLink = setContext((_, { headers }) => ({
  headers: {
    ...headers,
    'x-api-key': config.API_TOKEN,
  },
}));

const errorLink = onError(({ operation, response, graphQLErrors, networkError }) => {
  if (graphQLErrors) {
    graphQLErrors.forEach(({ message, locations, path, code }: any) => {
      errorLog(
        `GraphQL error, operation: ${operation.operationName}, variables: ${JSON.stringify(
          operation.variables,
        )}, message: ${JSON.stringify(message)}, location: ${JSON.stringify(locations)}, path: ${JSON.stringify(
          path,
        )}, code: ${JSON.stringify(code)}`,
      );
    });
  }

  if (networkError) {
    errorLog(
      `Network error, error: ${networkError}, operation: ${operation.operationName}, variables: ${JSON.stringify(
        operation.variables,
      )}, response: ${JSON.stringify(response)}, code: ${JSON.stringify(
        get(networkError, 'statusCode'),
      )}, graphqlerrors: ${JSON.stringify(graphQLErrors)}`,
    );
  }
});

const link = ApolloLink.from([errorLink, authLink, httpLink]);
const cache = new InMemoryCache();

export const client = new ApolloClient({
  link,
  cache,
  defaultOptions: {
    query: {
      errorPolicy: 'ignore',
    },
  },
});

then to fetch the data and persist it on the user device, I use mobx-state-tree, it looks like this:

fetch: flow(function* fetch(): any {
  self.isLoading = true;

  const { data, loading } = yield client.query({
    query: QUERY,
    fetchPolicy: 'network-only',
  });

  if (!data) {
    self.isLoading = false;

    return;
  }

  try {
    self.now = NowModel.create(data);
  } catch (e) {
    errorLog(`Cannot create NowModel: ${e}, ${JSON.stringify(now)}`);
  }

  self.isLoading = loading;
}),

Alright, that was the introduction, let's talk about the problem now.

I tried the app on multiple devices, in release mode, and I didn't have any issue. I'm using Sentry to get issues reports, and I received a lot of error related to networks issues (and with other queries):

Network error, error: TypeError: Network request failed, operation: myQuery, variables: undefined, response: undefined, code: undefined, graphqlerrors: undefined

I also checked logs on the server and I don't have any issues or errors on the requests, they all respond with status 200.

Things that I'm suspecting it's that it could be related to fetchPolicy: 'network-only' or errorPolicy: 'ignore' without being sure. Could it also be that when the app goes into the background the user device, it goes into a weird state for the graphql client? or could it be that the user is trying to load data when he is not connected to the internet?

thanks in advance,