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Just to clarify a bit more on the situation, are the cookies and cache cleared from the PlayStation 5? (Settings > System > Web Browser)
Are Final Fantasy XIV and the PlayStation 5 fully updated?
Have you attempted to power cycle the router(s)/modem(s)? (Unplugging the device and leaving it idle for 10-15 minutes)
Are there any others on the network when playing Final Fantasy XIV?

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