As already noted, that's where the dishwasher salt goes, to soften the water [or more specifically to backwash & refresh the resin bed* that does the actual softening]. Without salt the resin will stop working after a few weeks, allowing limescale build-up & a less-effective wash cycle.
Just refilling the salt will not make the smell go away, and in fact the smell is quite pervasive & difficult to shift. It's commonly described as smelling like a wet dog. You are unlikely to remove it in a single cleaning session. It's also likely to be clung to your dish-ware, which will also need more than one wash cycle to de-odour.
The decay & consequent smell isn't starting in the salt reservoir, but if it's not been refreshed in a long time, it could work its way into there, where it will essentially stagnate further. Bear in mind almost nothing can live in a highly concentrated salt solution… but yours is missing. The salt reservoir isn't an area that gets fully flushed through with clean water every time a wash cycle runs, it's little more than a 'drip feed' to the water softener's resin tank, meaning it will stagnate if there's no salt.
There seems to be some confusion in comments as to how a softener works, or what route water takes through it. The reason I say below to sluice it as best you can is that is is not a constant flow-through tank, it is only used to refresh the resin beads in the softener itself. Most of the inlet water flow goes straight through the softener, not through the salt reservoir, so trying to rod the reservoir without disassembling is a fruitless task.
This is what a typical softener looks like…
& here's an explanation of how one works - How does a Dishwasher Water Softener Work? (Image Guide)
Sluice the salt reservoir out as best you can with fresh clean water, so at least the water in there is 'new'. You may need to resort to a sponge & bowl/bucket to stop the bottom of the dishwasher flooding while you're working.
Don't use bleach or other harsh chemicals - the resin beads used by a water softener can be damaged by bleach. Some beads can stand it, some cant - & we don't know which type are in there.
Also, avoid bleach on anything made of stainless steel - Does Bleach Affect Stainless Steel? Our Research Found This - you may get away with a small amount, once or twice, but it can make even stainless steel rust. It can also stain the surface - which is not so much of an issue cosmetically inside a dishwasher.
Refill with dedicated dishwasher salt, not regular kitchen salt. This will make a lot of the water spill back over into the bottom of the dishwasher - this is fine. When full, wipe the screw thread before screwing the lid back on, to save the thread from becoming clogged and preventing the lid from sealing properly. Sponge out any spilled salt, or immediately run the machine on a short cycle, empty.
Buy a dedicated dishwasher cleaning product - this covers products specific to the UK market, so you may not find them all everywhere Best Dishwasher Cleaners (2023 UK). It also includes a cleaning & deep cleaning guide at the end of the article, and covers what types of non-specialist products are safe to use - vinegar, lemon juice, bicarb etc. Some specialist cleaners can be run as part of a regular dishwashing cycle, but the stronger ones are best run in a dedicated cycle, in an otherwise empty machine, on the hottest setting.
Aside from the smell, we can see quite a lot of limescale buildup in the dishwasher, so it might be necessary to run two cycles with the cleaner product. Buildup may be significantly worse in pipes & areas you can't see. Limescale is the killer of many washers & dishwashers.
A fully-functioning water softener will keep the machine limescale free & also improve the wash quality.
In warmer weather [maybe above 21°C or so indoors] you're going to need to run a cleaning product every month. Your nose will tell you when you need to do it again. Once you've re-established a regular cleaning routine, you won't need to use the expensive cleaners so often, you can get away with the cheaper in-wash cleaners.
Also note that cleaning involves removing the strip seal around the door as well as the obvious filter mechanism in the drainage sump. All sorts of unattractive gunk can get stuck in the seal.
*See What is Water Softener Resin? (The Only Guide You Need)