In theory you can change the mailing address of your bill using this online form
. No idea if it works but perhaps worth a try before you come? You will need to scan your passport and the paper they issued at the tax office showing your ΑΦΜ when you registered. Set your scanner resolution not too high or the file size might be too big. I would guess they will accept pdf or jpg but they don't actually say. I would imagine there will be a problem if you have renewed your passport since you started the contract as they will have your old passport on their files.
At the very least it will get you in touch with them. Another route would be to phone +30 211 211 0770 from abroad or 11770 when you get to Crete. Alternatively you could email them with a general enquiry about visiting the store at firstname.lastname@example.org
If you want to be more specific you could use their contact form
but that pretty much works on the same principle as the online change of address.
For some stores, not Rethymnon, they have an online system for booking an appointment so at least the stores seem to be open at the moment. Given the rise in infection rate how long that will continue is anyone's guess. I hear lots of rumours about another lockdown starting in early September but how accurate they are I have no idea.