Do your existing modems broadcast two separate Wi-Fi network SSID identities e.g. OPTUS_XXXXXX (2.4Ghz) and OPTUS_XXXXXX_5Ghz (5 Ghz) or just one common identity for both Wi-Fi networks - OPTUS_XXXXXX.
Are drop outs occurring from devices that use the 2.4GHz Wi-Fi Adapters, typically legacy units such as TVs, some Tablets and older PCs or are they occurring from 5 Ghz enabled devices which operate under lesser distances than the 2.4 Ghz devices.
Of the five modems, what is the model of the latest one?
Not sure if any recent issues of updated firmware have changed functionality with this modem, so I may be out of date with information.
The F@ST3864V3AC usually comes with Beamforming Transmission and Beamforming Reception ENABLED as Default. These settings are well intended but can cause issues for some user environments where devices don't support the function. When enabled, the modem changes the standard spherical area coverage of 5Ghz Wi-Fi signals to a focused one. Unfortunately some Wi-Fi adapters don't support the feature resulting in poor in room Wi-Fi coverage for their adpaters.
You may see some improvement if you DISABLE both features. If you haven't done so to date, you can connect to the modem by typing 192.168.0.1 into your web browser, use the admin and password codes on the print label of the modem to enter the Advanced Settings Menu, select Wireless , select wl0 for the 5Ghz, select Advanced, go to bottom of the very long page and DISABLE the last two Beamforming Settings and then Apply/Save.
Check the 2.4Ghz page as well - it is under wl1 - the Beamforming settings there should already be DISABLED in default.
Disabling these two settings also seems to disable a hidden Bandsteering function in this modem, the benefit of the disable action in this modem is that it also provides two separate Wi-Fi SSID identities for the 2.4Ghz and 5 Ghz Wi-Fi Networks. Some users have complained that their 2.4Ghz devices can't connect to the modem when Beamforming (and the hidden Bandsteering function) is enabled.
The modem that replaced the F@ST3864V3AC was the F@ST3864V3HP (High Power Wi-Fi) - unfortunately it is not possible to Disable or Enable the Beamforming feature in it - the drop down box only gives you a choice of Single User or Multi User Beamforming. It broadcasts both SSIDs as a default configuration ( assume Bandsteering is disabled in its hidden menus).
The F@ST5366TN-A replaced the F@ST3864V3HP.
Hope this helps