cannot upgrade or downgrade from x.83.0.55 firmware

We are using 3CX and have recently updated 3CX to version 16. We have several Yealink phones that are on version x.83.0.55 firmware and 3CX states that this version is unsupported. When I attempt to update the phones to a more current firmware, I get a firmware update failed message. To troubleshoot the issue, I have tried both upgrading and downgrading the firmware but, it fails every time. Phone models that we have this issue with are T41S and T21P_E2.

Anyone else have this issue (and possibly a solution)?

Thanks.