Did you want to try and uninstall Opera and then remove Opera stuff from the registry?
I'm willing to give some registry editing shot, I have a feeling that is where the problem is.
Never had a problem with the ISP before, and actually just swapped out the router the other day.
Delete Opera keys under "HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet".
Delete Opera strings from "HKEY_LOCAL_MACHINE\SOFTWARE\RegisteredApplications".
Do the same for those Opera keys and strings under "HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node".
Delete Opera strings if they're in "HKEY_CURRENT_USER\Software\RegisteredApplications".
Delete Opera entries in "HKEY_CLASSES_ROOT\Local Settings\Software\Microsoft\Windows\Shell\MuiCache".
Delete Opera entries in "HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\Shell\MuiCache".
Delete launcher.exe entries in "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ApplicationAssociationToasts".
Delete Opera entries in "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Search\JumplistData".
Delete Opera entries in "HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Compatibility Assistant\Store".
Delete Opera enttries from "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Compatibility Assistant\Persisted".
Delete Opera keys in "HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall".
Look for any "Opera Developer" and "Opera Beta" keys and remove those too.
Look for any of those keys/entries under any of the other users listed in the registry and remove those Opera ones too.
In Explorer, delete "C:\Program Files\Opera", "C:\Program Files (x86)\Opera", "C:\Users\yourusername\AppData\Roaming\Opera Software", and "C:\Users\yourusername\AppData\Local\Opera Software". Delete any "Opera Developer" and "Opera Beta" folders too.
Delete any Opera files and folders in "C:\Windows\Temp".
Delete all the files that you can in "C:\Users\yourusername\Appdata\Local\temp".
Delete any Opera folders in "C:\Users\yourusername\Appdata\Local\programs".
Opera should then be pretty clean from the system. Then, download the Opera installer, launch it, click "options", set "install path" to "C:\Program Files\Opera", set "install for" to "all users", uncheck "import data" and anything else you want to uncheck and then install. Then try Opera in its default state.
Now, you could use a registry cleaner to do the registry stuff first and then check for leftovers. But, this way, you can manually check everything you're going to delete.
If after doing all that and reinstalling Opera, if it still doesn't work, you might try a registry cleaner. CCleaner can do some cleaning, but there's a better one that often fixes issues. I just can't remember the name of it. Maybe some others have some suggestions.
@aspiring-squirrel I have that too and I think it's legit. See https://forum.adlice.com/index.php?topic=3340.msg7567#msg7567 for example.
OK, went through registry and disk directories, also used Auslogics Registry Cleaner ... No change.
@aspiring-squirrel Okay. Will think of some other things to try later.
In your normal Opera and in standalone installations you try, goto opera://extensions and click "all" if you haven't. Perhaps some extension is getting auto-installed somehow.
(or wherever you have Opera installed).
launcher.exe "--user-agent=Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36 OPR/57.0.3098.76"
Then see what Opera gets detected as. Is the OPR part still stripped? If so, does everything else match or is a different version of Chrome and AppleWebkit detected?
launcher.exe "--user-agent=Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0"
Does Opera get detected as that or does it still come up as Chrome?
Doing this may reveal something.
Just for addons.opera.com, ctrl + shift + i to open developer tools. Switch to the Network tab. Click "disable cache". Click the 3 dots at the upper right of the developer tools and undock to a separate window. Click the 3 dots again, goto "More tools" and then "Network Conditions". Then, at the bottom, uncheck "select automatically" for "User agent". In the drop-down for the User Agent, set the user agent to Opera for Windows for example. then, ctrl + r or F5 to reload. Then, switch back to the addons.opera.com tab in the main window. Does Opera get detected right then? If not, try Opera for Mac instead.
Again, maybe this will reveal how the user-agent is getting changed on you.
You said you checked for malware. But, any anti-virus software or security software or firewall software that might be interfering?
launcher.exe "--user-agent=Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36 OPLR/57.0.3098.76"
Notice the "L" in OPR. If the OPLR doesn't get stripped, then whatever perhaps whatever is causing the issue is looking for opr specifically.
Of course, maybe we're still missing something obvious and simple.