The speaker probably needs to be upgraded. You should be getting a prompt to “Upgrade” from the S1 app and all you need to to is tap update and follow the prompts.
If there is no prompt, try resetting the speaker again, but connect it to your router with a cable. Open the S2 app and start the setup process again. it should find the speaker and allow you to upgrade the speaker to work on the S2 app.
Getting another One is what I would do first, then a Sub Minis to complete the stereo pair.
There’s no prompt in any of two apps. Have tried to connect the S1-SL to the router with an RJ-14 cable and had no success. However, there was a switch and a long data cable between S1 and the router, and when connected by wi-fi, a Deco mesh device working on wi-fi 5.
Keep digging!