Recently setup BT Smart hub2 and cannot get a couple of apps so I bought Roku Express which won`t connect through WIFI , can I use ethernet cable for the Roku while using WIFI for broadband/tv .
Roku Express doesn’t have any Ethernet connection. They are WiFi only as far as I can see.
could use HDMI to ethernet converter , if router would take both broadband cable for WIFI and ethernet for Roku
I have no idea about Roku devices but a router such as the BT hub can have as many Ethernet and WiFi connections as you like simultaneously ( well up to 252 IP addresses in reality)
You would need a USB to Ethernet converter, not HDMI. Roku support pages offer some suggestions for suitable models.