Maybe try wiring the Sonos Amp temporarily to your router/LAN and see if it will show/setup in the App - if it’s been factory reset then use the popup card to setup or use ‘add product’.
For WiFi purposes later, the standards required are shown here (I recommend using 802.11 b/g/n and WPA2-AES personal):
https://support.sonos.com/en-us/article/supported-wifi-modes-and-security-standards-for-sonos-products
The Sonos controller mobile device used for setup needs to meet these requirements…
https://support.sonos.com/en-us/article/sonos-app-requirements
Ensure the App can access the LAN and if the mobile is using security siftware, such as a VPN client, site blockers etc. then disable those things temporarily to assist with the setup.
If the issues persist then it maybe best to reproduce the issue(s) seen and then immediately submit a Sonos system diagnostic report from within the Sonos App, note it’s reference and contact/chat with Sonos Support Staff via this LINK and see what the Staff can perhaps suggest to help you to resolve the matter.