Rubber or foam rubber?
Old-school rubber surrounds last, foam does not - imo it makes speakers semi-disposabl, except that you can get "re-foam" / surround replacement kits.
Problem is UV light, and drying out /evaporation/"de-gassing" of plasticisers. There is stuff that helps with preserving and restoring vinyl and plastics, I have it for motorbike seats and plastic parts...not a consumer product, comes in quite large containers. ...I can't remember the name...
...303 Protectant. I see it's now available in spray bottles, even Amazon.