Can't load 4kBasic from serial port

[M D]

I've been tearing my hair out for several days now trying to figure this out. I don't understand why it doesn't work! I followed the instructions exactly from here:

https://adwaterandstir.com/2017/05/30/msbasic/

But it simply will not load in the tape data using the serial port. That's the only thing I did different from the instructions, because that is how I connect to the Altairduino (not via USB). I've been working with Google Gemini to try and figure things out. It's helped me at least show that I am actually connecting via the serial port after a bunch of configuration changes, and we even rewrote the bootloader from scratch like Paul Allen did in the plane to Arizona 50 years ago, and it just hangs after the first byte.

I don't know what to do here. It's really bothering me, as I've been requested to make a video of the process by some friends. I feel like a failure. Good thing I wasn't the one trying to demo MASIC to MITS way back when. Micorosoft would have failed and history would have been destroyed. Any help would be greatly appreciated! before I lose my mind completely! Thanks!