window.ethereum undefined on mobile?

I built a web application calling metamask with code such as this one:

```
await window.ethereum.request({
method: 'wallet_switchEthereumChain',
params: [{ chainId: chainId }],
});
```
But on mobile, android chrome, window.ethereum is undefined, despit metamask being installed.
On desktop everything work as expected, any idea what could be the reason?

Do we have to do something special for mobile?

No repository files or tests are named. Start by reproducing the report in Android Chrome with MetaMask installed, comparing whether window.ethereum is present on mobile and desktop; done means the cause and the required mobile behavior are identified and covered by an appropriate fix or documented resolution.

Written by the indexing model from the issue text.