Good morning and thank you for posting on the Final Fantasy XIV Technical Support Forums.

It's difficult to discern where this issue stems from so we have a few different suggestions you can try:

Try connecting to your router with an Ethernet cable as opposed to Wifi. This can make the connection much more stable.

Make sure your anti-virus/firewall has the appropriate exceptions for Final Fantasy XIV (you can temporarily disable them while you try to connect as well).

Check the following Port Ranges on your router/modem to ensure that the following Port Ranges have been opened:
▼TCP
54992 through 54994,
55006 through 55007,
55021 through 55040
Your ISP may forward these ports for you.

We hope we can be of assistance in helping you enjoy the exciting world of Final Fantasy XIV!