For the torque converter code (P0743), it usually indicates that the transmission fluid level is indeed low, so that could be the culprit. Make sure to check the transmission fluid and top it off if necessary. You should be checking the fluid with the engine running. The transmission can be in park or neutral. Ideally, with the fluid hot, you should be at the hot full mark.
As for your code reader not powering up, that could be a couple of things. First, ensure the connection is secure and the OBD2 port isn't damaged. Sometimes, there might be blown fuses that can affect power to the port. If you have a multimeter, you can check if the port is receiving power. That's where I'd start at least.
It might be time to consider taking it to a competent mechanic. There's no harm in that.
