replacement router - fibre vs adsl

Hi talktalk

have been sent replacement router for DSL-3780, however replacement is HG-633 with ADSL port for internet side rather than fibre (presented from bt as ethernet connection.

have contacted support but they are advising to plug ethernet into adsl connection (does'nt fit) and don't seem to be getting the idea. 

could you advise what the replacement for the DSL-3780 router is please, or why the setup guide says use ADSL when it's fibre. 

have spoke to techie and they dont get it

Thanks

Stuart