Nokia 8110 4G runs on KaiOS which does not have the WhatsApp support. Nokia management has updated that they are working on the same but there is no definitive promise made for the same.
No. Exactly we cannot say that Nokia 8110 4G is a waterproof device. But it is IP52 certified. Means it is splash proof. You are protected up to some extent by water.
Yes, Nokia 8110 4G does have the FM radio.
Nokia 8110 4G comes with 512MB RAM and 4GB internal storage. You will get practically unlimited storage for SMS.