Why can't I connect to my Amazon Lightsail instance using {ip-address}+{port-number} in my browser? Firewalls seem to be set correctly

I have set up a new Amazon Lightsail instance with the intention of running a few things such as a VPN and Resilio Sync server. I have also set up an instance (or "droplet") with Digital Ocean to compare the two services. So far, although DO has generally been easier to set up and use, they seem to have sold me a blacklisted IP address which will cause issues for me; and Lightsail seems to have better security out of the box among other things, so I would prefer to stick with Lightsail.

However I am hitting some real problems which I hope the community here might be able to help me with. I am still relatively new to Linux so quite possibly I'm missing some basics. The first problem I've encountered is in the process of trying set up Resilio, although I suspect it is a more general issue rather than anything actually stemming from Resilio itself.

I have installed Resilio on Lightsail (following these instructions) and have confirmed the service is up and running without issue. In the /etc/resilio-sync/config.json file, I have set the following:

{
    "storage_path" : "/var/lib/resilio-sync/",
    "pid_file" : "/var/run/resilio-sync/sync.pid",

    "webui" :
    {
        "force_https": false,
        "listen" : "{ip-address-of-my-vps}:8888"
    }
}

All quite straightforward. Now, when I got to this stage on my Digital Ocean droplet, following all the exact same steps, I was able to simply connect directly to my droplet by navigating to {my-ip-address}:{port-number} in my browser, as specified in that config file; and voila, it brought up the Resilio web GUI.

This doesn't work in Lightsail. I am at a complete loss as to why.

1. I have set up a static IP address and attached it to my instance. I can assure you I'm entering it correctly in all places.

2. I have set appropriate Firewall rules in the two places I am aware of: UFW on the instance, and Amazon's Firewalls enforced at the console level.

My UFW rules look like the following:

In my Lightsail console, in the network tab, I have also set the following rules:

(I have also tried with the following):

Please can someone more knowledgeable than myself explain/suggest why I am not able to connect directly to my instance using {my-ip-address}:{port-number}?

Thank you.

Edit: Both my DO and Lightsail instances are running Ubuntu 20.0.4