You'll have to do this on your Vultr Ubuntu server.
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The first place you can look is in your Vultr control panel, to see whether there are any firewalls enabled there. You will need to open up access to ports 80 and 443 (TCP).
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The second place you can look is on your Ubuntu server. A good place to start is to run:
ufw status
If it shows that the firewall is enabled, then you should allow those ports:
ufw allow http ufw allow https