Cold air from the attic is freely circulating under the floor of the cold room. Plug those openings with insulation.
If you can stuff 12in deep in to the space, that would already be excellent. Make sure the insulation is stuffed tightly to keep itself in place and to choke off air circulation. No need to use spray foam, just rock wool or fibre glass batting is fine. (Don't use rags, old mattress foam or other shortcuts, house insulation is fire rated) They say "don't compress insulation", but that is fine here. Squeeze it or fold it over as needed to get it jam tight.
Additionally you can close those openings with strips of wood, like plywood. Floor joist space does not have to breathe. Leave gaps for the cables so they do not get pinched, and stuff the gaps with left over pieces of insulation. Frankly, this is extra work that won't add much after you have stuffed the openings.
This assumes there are no other openings on another side, but if that's the case, this would also stop or limit drafting.