Where do you see workgroup on the router? I don't see it

If you look in the router you won't find it .
I think it is hard coded into the router
and could only be changed with a Firmware update ?
If you use A network scanner. "WORKGROUP" is listed as the Domain it belongs to.

There are possibly ways to do this ;)

Go look at some of my posts , you need to tweek a config file and re upload it , similar to getting true telnet " NOT TP Link Telnet " to work on these.

This is done to allow Real Telnet / changing DynDns settings etc , so I assume the workgroup could be changed just as easy.