How to force Network Manager to rescan connections?

sudo iwlist interface scan

Where interface is the name of your wireless card, e. g. wlan0

Try

nmcli device wifi rescan

and

nmcli device wifi list

to see available networks

In Ubuntu 16.04 and newer, restarting NetworkManager with sytemctl works (at least after scanning the networks manually with iwlist):

# Find the name of the network interface, e.g. wlan0 or wlp3s0
ip addr show      

# Scan for WLAN networks (replace wlan0 by the correct interface)
sudo iwlist wlan0 scan

# Restart Ubuntu’s Network Manager so it reloads the Access Point list
sudo systemctl restart NetworkManager