Hello Bill @phi148,
Yes, the filter option is not implemented in iosc mobile.
There is only the simple manufacturer selector in catalog_mb and then on the new_products page the combination of the manufacturer and category select menu which is the equivalent of the filter option on the main site.
I tried this on my standard develop 2.3.4 store, but it doesn't give an 404 not found error. It just ignores the filter parameter and shows all products or all products of a manufacturer, if selected.
To solve this issue you could strip the filter parameter in the mobile redirect class or add a rewrite rule to .htacess.
Otherwise catalog_mb would need to be recoded including a category select menu and using the filter parameter.
A lot of job and like already stated, I went Bootstrap and will not continue adding new features to iosc Mobile.
If you can do it and find a good solution I would be happy to add it to the package.