Use the little tablets kids clean orthodontic plates in & older folks clean dentures in.

Eg: https://www.chemistwarehouse.com.au/buy/7187/polident-denture-cleanser-fresh-active-tablets-36 or any similar "denture tablet" equivalent.

Pop a few inside, with boiling hot water filled right to the brim, and leave for half an hour on your draining board/on your sink (it likely will bubble & overflow a bit).

Magic:)