Upsells need to be created in Rezlynx first as ‘Products’.
If you were using the DBM before ecommerce platform then hopefully you’ve got the upsell products already created, but if not, they need to be created in Rezlynx first before they can be set up for the RoomlynxWS channel (ecommerce platform).
Once you've got the upsell created and exposed to Roomlynx, you'll then need to set up the description and imagery, by going to Distribution > Upsells and then edit each one.