I'm confident @Mkrtich will help you figure this out.
Hi @Android5 . You have a few unexpected peculiarities happening here and I am not sure if the modem may have technical issues or not at this stage. Just a few questions
1. Have you disabled WPS in the Wireless Settings of both frequencies in their respective pages and Saved the changes; wl0 = 5Ghz and wl1 =2.4 Ghz.
2. I have found with handling the modem, it is very easy to unknowingly turn OFF the Wi-Fi button on the rear panel (up from the power cord); sometimes when I have no Wi-Fi from a device, I have realised by looking at the unit, that I have hit it OFF as there is no Wi-Fi LED ON. A modem Restart seems to bring an Wi-Fi OFF status, but I am not sure if that is because of my fingers hitting the ON/OFF button on the rear during the Restart process.
3. The modem may be set broadcast a common SSID to both frequencies - are you seeing two SSIDs - e.g. OPTUS_AABBCC (2.4Ghz) and OPTUS_AABBCC_5G (5 Ghz). If you wish to see two SSIDs and link your devices to preferred bands, this is done by Disabling two features within the very long wl0 and wl1 pages in Advanced - right at the bottom, BMTR(BFR) and BMR (BFE) and Save. This has assisted some people who could not link some of their 2.4Ghz devices to the modem - their devices may see the modem but the modem's transmission back to the device was compromised by the setting that provided a common SSID to give the 5Ghz a priority.
4. Have you made any changes to the DHCP Address range setting within the Modem? When you connect the Windows PC, can you look at the Adapter Properties to make sure it is Enabled and see if the Wi-Fi Adapter is OFF. Then do a Properties look up on the Ethernet Adaptor for its IP address, SubNet Mask, DNS Primary and Secondary Servers - the PC should be getting this information from the Optus Modem showing its Gateway Address as 192.168.0.1.
5. Can you disconnect the PC from the modem, turn PC OFF, wait 60 seconds, Restart the Modem, reconnect LAN cable to modem and then turn on the PC. The Modem should allocate your PC a fresh IP address. Validate this is so by looking at the Adapter Properties. If you can't access the Internet from the PC, can you try another Browser?
Is failure to connect to the Internet on the same site or all traffic? If on all traffic, can you alter the DNS Server entries in the Windows PC Adapter to Google's 188.8.131.52 and 184.108.40.206 to see if the PC connects?
6. Android, - Disconnect or Forget Wi-Fi Network, then try a Soft Restart in case that clears out any issues with connectivity and then Reconnect with you new Wi-Fi preferred SSID.
7. I suspect you would not be able to directly connect to your WAN address as it is outside the range of 192.168.0.1 and you may have been directed back inside to the admin screens. Another cause could be that your Web Browser Data Cache may have a corrupted entry - sometimes Clearing Out Data Cache and History from the Browser gives it a fresh start and fixes the return to admin screens. conditions.
Hope this may be of assistance and let us know what you have discovered. Optus should also be able to remotely access the modem to see if any technical issues that arise within it require replacement of the modem.