I've built a website and have used certbot to create SSL certs.
The page loads fine on every system I've tried it, except in Chrome on my phone.
In Chrome, on the phone (Android), I get NET::ERR_CERT_AUTHORITY_INVALID
In FF on the phone it loads fine. In Chrome or FF on my tablet it loads fine. In Chrome or FF on laptop and/or on desktop it loads fine.
What's up with Chrome not playing nice?
As per @dave_thompson_085's comment, the answer to this is that my phone is running too old a version of Android to have LetsEncrypt in its root stores.
Now that LetsEncrypt has chosen not to renew its cross-sign, Android versions older than 7.0 don't consider LetsEncrypt certs to be valid.
In case anyone else runs into this issue, the reason Firefox DOES work on old Android versions is that it uses its own root store, rather than the android OS's store.
Hopefully Chrome might incorporate this upgrade into a future version, but for now it's just a shortcoming in Chrome.