nmtui is limited, for more settings use nmcli

nmcli connection show

In the following use case the raspberry is connected via eth0 to mobile router having low band width access to the Internet but therefore physically portable and cheap. However when moved to the office for development low bandwidth is an obstacle. Turning on to fast wlan0 would be a booster but the Raspberry would still use eth0 since it has a lower metric value. It would however switch to wlan0 when the eth0 cable gets removed. nmtui can not do this but nmcli can!

To give priority to wlan0 its metric must have a lower value.

nmcli connection show to see the names of the connections

sudo nmcli connection modify "<wlan SSID>" ipv4.route-metric 50 set a low metric and therefore high priority

sudo nmcli connection modify "Wired connection 1" ipv4.route -metric 200 set a high metric and therefore low priority

sudo nmcli connection up "<wlan SSID>" takes the modification

sudo nmcli connection up "Wired connection 1" takes the modification

To see the result: ip route show

default via 192.168.1.1 dev wlan0 proto dhcp src 192.168.1.161 metric 50 default via 192.168.20.1 dev eth0 proto dhcp src 192.168.20.5 metric 200

curl -s https://ifconfig.me will show the wan Internet address of the wlan0 router and uses high the speed connection.

sudo ping -c3 192.168.20.1 will still find the eth0 router

Observe the files in /etc/NetworkManager/system-connections, they contain now the new metric values.

Disable wlan0 and curl -s https://ifconfig.me will show the wan Internet address of the eth0 router and use the low speed mobile connection.