If the certification generation/usage is anything like it is anywhere else in the computing world (coming from the software industry, all this stuff works along the same lines) this is the rough situation:
The payment terminals probably use mutual TLS, or something similar. This means that both the terminal and the server it connects to must present certificates to each other to prove their identity, to make sure that they are talking to who they think they are. However, the certificates that they present each other expire after some time. It is totally possible to renew these certificates, and if process is in place, these certificates are just renewed before expiry, and no one notices.
However, sometimes, for whatever reason, certificates fail to renew and can't be renewed past a certain date. Since the terminals can't authenticate back to base to get an updated certificate (since it has expired), they sit there unable to connect, "bricked".
To fix this problem, you just need to replace the certificate, in this case, in person.
I have had this exact problem and heard of plenty of other software operations having this problem. It's the sort of thing that happens once, usually the first time your certificates expire, and then you 10000x triple check that you have monitoring and alerts in place to make sure that this never happens again.
I can hear the nay-sayer's saying "if this was a reputable operation, this would never have happened". This is a really common issue, and has hit big players like
Microsoft and
Spotify, just to name a couple.
Just my 2c, doesn't worry me from a technical POV. Excited to see further growth here. DYOR etc
(20min delay)