When I do it, I get a message “Couldn’t connect” for whatever network I select (as shown in the screenshot in the first post of this thread). Note that mobile data works for me if I leave it on “Automatically select network”, but connecting to the same network manually fails. The reasons for why I personally need to do this, I’ve described in another post:
I also just received a reply from Fairphone support in my ticket that I opened last week:
Our Software team is aware of an issue, which makes it impossible to manually connect to a specific network.
After an extensive investigation, our team discovered the root cause of the issue. A fix will be included in a future software update. We expect that this update will be available at the start of October so keep an eye out for that notification.
They also mentioned that they will look into whether they can make the fix available through the beta testing program, but my understanding from their message is that it is not in the current beta version.