Goto opera://about and take note of the Browser Identification string.
Then, for testing purposes, right-click on "C:\Program Files\Opera\launcher.exe" and choose to create a shortcut on your desktop. Right-click the shortcut and goto "properties". On the "Shortcut" tab, edit the "Target" field to set a custom user-agent string. You want all of the string except for " opr" (notice the space) and everything after it. For example:
"C:\Program Files\Opera\launcher.exe" "--user-agent=Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36"
Launch Opera like that and see if it works. If it does, then you know you need an extension to do that. There's this one that does it for all sites. There are others that can do it on a per-site basis. Some of them haven't been updated to use modern strings though.