Mtp is your friend, here is hiw I do it on my devices (samsung a23 and both opensuse and arch):
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I connect my phone to my pc. Then select mtp in the phone notification.
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Start my file manager (dolphine or whatever) and access my phone storage from there.
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Make sure to allow the notification on the phone asking if you want to sahre your storage with the external device.
Ddns is your answer, check your router and see what it can support or just go with whatever you feel good for you and install their updater on your server.