On 30 September 2021, LetsEncrypt's DST Root CA expired invalidating a lot of older browsers and applications that relied on the certificate.
Unfortunately, if your customers and clients are using older devices, there’s not much you can do when they can’t reach your website.
If you have a lot of customers that are in this situation, then your best bet would be to purchase a certificate (not from Let's Encrypt) and update your certificate on our system.
If you have questions, just let us know at Support and we can help get your certificates changed out.