The modem has wireless AC (5GHz) capability. It needs clients with AC capability - phone, PC, notebook, etc. Please check yours.
I have the 3864AC and have NO issues with any of my AC capable clients.
Yes some devices are not 5ghz capable, however, all that my devices that are ie, latest iphones etc, it always comes up with the same message, wrong password. Despite tech changing my passwords many times, resetting etc. As I have said before I have spent many tens of hours of my time on the phone to tech support and the constant thing I hear from them is "but that should work!" For whatever reason it does not work. I have heard it seems that this problem happens more frequesntly with ex cable broadband subscribers.
There are, potentially, two issues.
First. A Google search indicates that the 'password' issue with Apple devices is not restricted to Sagemcom routers. Recommend you perform such a search and implement the recommendations that have solved the issue, until you find one that helps.
Second. Individual nations have regulations regarding wifi frequencies (channels). The 3864AC has two settings for the 5GHz band - Auto and 36/8. My 3864AC defaults to Auto, and it defaults to channel 149. I recently upgraded my desktop PC's AC adapter (USB dongle). After installation, the dongle could not 'see' any 5GHz SSID. I changed the Sagemcom's setting to 36/80 and all was OK (I was able to make a registry change that solved the channel 149 issue - Google search provided the solution). My point is that some clients sold here do not have the required, wifi settings for Australia; a good check is the device's 2.4GHz channels - 1-13 (AU) or 1-11 (US).
Yes, regarding the iPhone issue. I had almost forgotten this. Damn and blast Apple, my wife still persists with hers and she is constantly losing her 5GHZ connections with any WiFi she is connected with. Her S5 Samsung is always connected and my new HauWei is even better, I mean the speed of the connection. When I had an Apple phone it to was a nightmare to maintain the connection wherever I went, finally seeing the advantages of anything but Apple I found that in an airport I was connected immediately to the free terminal WiFi and with a good signal strength. So, yes, the Apple phones do need a lot of cajoling to get them connected and then they are still not stable, so you may find you are using your phone data as well.
Just got off hte phone with tech support. Apparently Optus techs are upgrading the cables to cope with the high demand for NBN in our area, and this affects the 5GHz band. We'll see in a week or so.....
Greatadvice except the first hour-long phone call was spent trying to get the tech to understand that the Mac and the Phones were fine on 5GHz until a month ago - THEY weren't the problem. Second call was better. Tech claimed the cable fiddling at the exchange was upsetting the 5GHz band in MY modem, but should be a week or so. We'll see....
That change should be better advertised by Optus and its technical support; I suspect that it solves a lot of issues. The 3864AC defaults to auto which seems to default to channel 149. Channel 149 transmits at a much greater power than channel 36. I am in a quiet wireless environment and have great performance using 149. In a congested wireless environment, channel 36 probably has a better chance of good performance.
I recommend that Windows 10 users download the app wifi analyzer from the Windows Store - it's free. It gives a useful picture of your wireless environment and can be useful in solving issues. Unfortunately, Optus provide only the two settings for 5 GHz: I believe that more liberal settings would be extremely useful. BTW, all the Telstra wireless signals in my location use channel 36 - probably because of the potential issues with the higher power channels in congested wireless locations.
PS: Any device that happily works on (Sagemcom F@ST3864AC) 5 GHz channel selection 36/80 and not on 'Auto' has a wireless card/driver that will not receive all the Australian channels. That tends to happen when there is no country setup for the driver (defaults to somewhere else). That will happen if the Auto setting defaults to 149. Lots of info on Apple fora.