You need to set up a command the system form the iPhone that is not used to supply WiFi. Can’t really tell if that’s what you’re doing.

On the phone I use to connect I enable the hotspot option. With the second phone where I have the sonos app I add my speaker . Everything works until I am asked for the wifi password of my hotspot. Once asked the sonos app says a connection could not be established.

Odd behavior, I’d think that should work. I’d suggest you call Sonos Support directly to discuss it.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Hmm. Assuming the Sonos can ‘see’ the network. It conforms to the data in the Sonos Network Requirements FAQ? I’d also suggest using a 2.4 GHz channel, just in case. 

On the phone providing the hotspot do you have any unusual characters in the name?

You mentioned having to type the network name in manually because it wasn’t listed. Have you tried just using an alphanumeric hotspot name without spaces or special characters.

While it was 8 months ago, someone else was having problems due to an underscore in the SSID (network) name which was preventing their speaker joining

While the standards were changed many years ago to allow basically any character to be used, some devices still don’t handle it well.