Unable to activate Lenovo tablet as a "secondary" Viber device
I contacted Viber Support several days ago. Eventually the answer came from Viber Support (Ian). The steps are like this:


Remove Viber from your tablet (I added this step).
Go to tablet's Settings  Display  Display size and change from Default (or anything else) to Small.
Reboot your tablet so its dpi will change.
Install and activate Viber on the tablet as a secondary device (now you will see the expected QR code).
Change the Display size back to whatever it was before.
To be sure, reboot your tablet and test Viber.


Note: Unfortunately, I cannot check the "Display size" of my old Samsung tablet, because I already did a factory reset on it.