tl;dr: use exact="search:order-model"
on your Results tag.
You're not using the Keywords filter here, but the Field Search filter. Hence, the search_mode parameter is not used; it's only used in conjunction with the Keywords filter. The Field Search filter is basically the native search:field_name
parameter on steroids, so you can mimic what you'd normally do with such a parameter.
For example, this: search:order-model="=m-200"
would look for exactly the value "m-200" in the target field. This is the same as using search:order-model="=mmodel="m-200" exact="search:order-model"
, which you can split up with LS into a form field and parameter on the Results tag. You've got the former param as a dropdown in your form, so you should add the latter to the Results tag.