This is the first time mpsantos has posted.
1st of all welcome to EOS forum 
If the router is on during Endeavour’s boot, I can see the SSID on the network manager and connect to it.
But if I turn the phone is off during boot, the SSID won’t show up. If I reboot, keeping the router on, I can see the SSID
I faced exactly same issue the 1st time I switched to Linux (Linux Mint) quoting "If I Disable & Re-enable wifi OR remove & re-connect wifi dongle OR when I try to connect to a different device then it says “connecting” for a while then notification popup that “connection failed”
Check if it helps: [SOLVED] WiFi Driver issue fix
As yours is driver: Realtek “rtl8821cu”, follow https://easylinuxtipsproject.blogspot.com/p/realtek.html#ID6