I have a Synology router. I couldn't get connected after dozens of attempts using my 2.4 GHz, even though it was named differently than my 5 GHz. I tried numerous things like a different device instead of my phone, disabling my 5 GHz network.
This worked in the end. I created a guest 2.4 network with a very simple password. That allowed me to connect immediately.
This worked in the end. I created a guest 2.4 network with a very simple password. That allowed me to connect immediately.