Issues playing Paul Wheaton's podcast on Apple Podcasts? Expired Certificate?

This isn't necessarily permies.com forum related but I was wondering if anyone had any insight.

I have an old iPhone 4s that I use as a dedicated podcast player (using the included Apple Podcasts app because no other podcast app can run on a phone this old.)

A few months ago I stopped being able to download or stream any of Paul's podcasts (old or new) using this device. This is what it tells me when I try (see attached photos):

Cannot Verify Server Identity
The identity of "richsoil.com" cannot be verified by Podcasts...

When I click the certificate "details" it says:

Not Trusted: Expired 2021-09-30, 7:01:15AM

I've tried manually "trusting" the certificate; still no go. I tried erasing and formatting the entire device and installing any updates via itunes; nothing. I've tried dozens of other podcast channels and none of them have issues. It's only this one.

Anyone else having any issues or is it just me and my dinosaur of a device?

Do you also get this error when you simply go to https://richsoil.com from that phone?

Do you also get the same error when you go to https://permies.com from that same device?

I get the same error when I go to both of those websites on the same device (safari), but I can click "continue" (first screenshot) or "Trust" (second screen shot) and it immediately loads the website (both sites). With Apple Podcasts, when I do the same, nothing happens (podcasts still won't load).

This might be a bit technical...

The old root certificate for Lets Encrypt Expired 9/31/2021.  
https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/

Any recently updated browser or operating system would have already had the new root certificate installed a long time ago. Your older phone likely hasn't been updated in quite some time.  If the root certificate expires(The older LetsEncrypt cert), then the child cert(richsoil.com) is effectively expired also. Regardless of its actual expiration.  

Root Certs are used to create the child/site certificates. When your computer/phone looks at a certificate, in this case richsoil.com, it checks the certificate's root authorization against the root certificates it knows about. Your old phone has the OLD LetEncrypt root certificate. These only come about by browser/os updates. The new root cert expires in 2035.

Note: "R3" you see in the "Issued by" info is the name of the certificate is used by "LetsEncrypt"

Thank you! That explains a lot. I assumed something must be outdated, but the fact that only Paul's podcast is affected made me think there might be more to it.