Difficulty accessing Apache2 webserver from external browsers

I've followed the instructions in the Linode guide Install LAMP on Ubuntu 16.04.

I can access my index.html and phptest.php from within my linode CLI using the links browser (using either the IP address or the domain name, both methods work).

From external browsers, I get ERRCONNECTIONTIMED_OUT.

When I ping my domain name from an external computer, I get a prompt ping response from the correct IP of my Linode.

I'm using Linode DNS, and in the zone for my domain name, I have A/AAAA records for both the IPv4 and IPv6 addresses. (I have no CNAME, TXT, or SRV records).

I have Reverse DNS configured for both IPv4 and IPv6 with the Target of my domain name.

systemctl status apache2

shows that apache is active (running).

What else do I need to check?

Thanks!