I just want to confirm: when you say you “notice” the bed heating up - are you actually feeling it, or just looking at the display?
I’m trying to establish whether power is actually going to the bed heater and hot end heater. If the bed/hot end are not getting warm, then you (likely) have a heater problem. If there is heat but it’s not reflected in the display, then you have a thermistor problem. The controller is a bit on the dumb side. It can call for heat but it doesn’t actually measure that current is flowing through the heating element. It simply reads the thermistor values and complains if the value being read isn’t what it expects. However, that does not distinguish between the two possibilities: that the bed was never actually getting heated in the first place, or that the sensor is sending back an incorrect value.
Here’s a thought: have you verified that you haven’t swapped the thermistor cables? That is to say, if you accidentally plugged the bed’s thermistor into the hot-end’s thermistor socket (on the motherboard or the wrong extension cable), then the bed will start to heat, but the controller would be accidentally reading the hot-end thermistor. That’s an easy enough mistake to make as on many models they are fundamentally the same thermistor using the same type of plug.
I should point out, if you weren’t already aware of it, that bed generally heats first and the nozzle only gets heat when the bed is up to temperature.