Ahh Apple is a bit different to Windows in regards to hiding or removing other networks from your list, basically it is not possible. So Fon will always appear in a list of available networks if it is near enough to register on your list. It shouldn’t be the default or even the highest ranked choice of networks in a list that hasn’t had a network moved to the preferred position ie the top of the list. I see you now have fixed the default choice so at least a better outcome, but I have linked a youtube video that shows you how to list the networks in order of your preference
These are just my thoughts as I’m not that familiar with Macs.
They normally list in order of signal strength in Windows and it may be the same for Mac thought on a Mac preferred can be dragged to the top of the list to make them first choice when connecting. If a Fon one is presenting before your own (if you haven’t made your one the preferred one so the listing is just in default order) you may have a neighbour or nearby hardware that is providing a stronger signal where you PC/Mac is located in your premises.
So to try and fix that you should be in default order of signal strength. Move your modem around or if it has external antennas then moving these to see if signal strength can be improved. This will involve moving the router a little eg if facing Sth try moving it so it faces a little more Westerly or Easterly and look at the result of your network listings on your Mac.
Keep changing the position or the router/antenna/antennae so that you see if you can get your Optus network to populate the top of the network listings. Then remake it the preferred network.
For you @val2 and those Mac owners who are unfamiliar with how to do this see this short youtube video:
You may get a better result by installing Ethernet over Power, WiFi boosters, range extenders, boosters or a WiFi Mesh system in your house if there is too much interference in your house between the router and your Mac. No WiFi signal likes going through wooden walls, steel structures, brick walls and similar stuff in a house.
The standard 2.4 GHz band (in routers this is usually b,g & n) will transmit further than the 5 GHz band (n, a, ac & ax) and will pass through the obstacles a bit better. However the 2.4 GHz is heavily populated with other routers nearby, microwave ovens , cordless phones and a myriad of other devices that cause interference and use this bandwidth so it will be impacted by that noise.
5 GHz is not as well utilised by these devices yet, so you have less electromagnetic signal clutter. 5GHz allows more data to be transmitted than 2.4 GHz if the area is clear of obstacles but 5 GHz will be more savagely affected by distance, walls and other physical obstacles.
Range extenders, Mesh, and Ethernet over Power devices remove much of both types of clutter caused network degradation.
For CHOICE advice, reviews on these types of devices visit the following (please note that some of the linked information at this site will be member only content):
I hope this is helpful.