Welcome to TPG Community!
We were unable to locate your account using your community details to look into it.
You are right, for 5 GHz connection the 4 meters is quite short for its range, however, we still need to consider the factors that can interfere with the WiFi signal. (e.g. any wireless device near the modem, walls, electromagnetic appliances, etc.)
For the 2.4 GHz connection, it is odd that your devices won't connect to it automatically, unless there's a high interference affecting this, which may explain why your 5 GHz connection is playing up.
Keep us posted if it's going to improve or not.