M2M vs normal SIM cards
There is no physical/electrical difference between the SIM cards. It is the economics of the network provider.

When a device connects to a cellular network it provides its IMEI (serial number of the device/modem) and IMSI ("number of the SIM card"). Each manufacturer has its range of IMEI numbers, so you can distinguish Simcom, Telit, Quectel, Samsung, Apple etc. just by looking at the serial number.

The operator can configure the network to disallow particular combinations. When your module connects, the network can see that "it is Digi" and look up the billing/plan information of the SIM card (based on IMSI) and decide - yes you can or no you can't connect. Maybe in the case of your network operator Digi products are treated as an M2M devices, while Simcom modules are not.

Many M2M devices send very little data per "daily smartphone standards", so the operator wants to squeeze as much money as possible by market segmentation.

Overall this issue is a pricing strategy.