This all depends whether the merchant choose to accept AMEX to begin with. AMEX comes with higher MDR (cost to merchant) vs Mastercard or VISA thus the merchant may choose to disable "accepting" AMEX with the merchant bank they sign up with. Payment terminal are just the hardware side but it is what is being configured at the backend that matters and this is transparent to the consumer or even the shop that you see the payment terminal.

Eg. A worker at the chain bubble tea shop likely will not know what are the arrangement with the bank as the arrangement happens at the HQ level.

May I know where to purchase Koncase ?
Googled and AliExpress came up and the price range between RM60-RM120.