Cant register lightsail instance as hybrid in Systems Manager

After updating Ubuntu in my lightsail instance, I'm now locked out via both the console and ssh. After some searching, it looks like a way to recover might be to create a new copy of my instance connected, as hybrid, to the AWS Systems Manager. However, I'm not able to do that. I'm following the instructions found here: https://aws.amazon.com/premiumsupport/knowledge-center/add-lightsail-to-systems-manager/

I've created a new Hybrid Activation. Made a copy of both the Code and ID. Taken a snapshot of the instance. Created a new instance from that snapshot, adding the code snippet (correctly updated with code, ID, and region) as the launch script.

But following this, launching the Fleet Manager never shows any instances. Any ideas what I can try to see why this isn't showing up.