My only solution is to use Firefox.
Firefox simplifies access to synced tabs:

Re: Sync tabs icon missing on sidebar

Hello, I'm using Opera (v68.0.3618.63) on Windows 10 (v1909)
After signing in to the Opera account and activating sync, I can see the Tabs icon in Sidebar:

But as soon as the app closes and reopens, this icon is no longer visible:

Please help me.