The original bootloader doesn't give access to the bootloader (keystrokes are ignored).
That's possible, and would be unfortunate. However most MT7688 U-Boot's I've seen have a 1 second timeout, requiring you to be carefully setup and quick about it.
reset buttons appear to have no effect either .
If you don't continue to get serial output after pushing it, it may be that the SoC is resetting but then failing to boot because something has left a stateful flash chip in a different addressing mode than the SoC is strapped to expect. You could also be getting the wrong reset input - the "wifi reset" is just a GPIO handled in software, and the PCIE reset would likely be irrelevant.
Typically the best procedure would be:
- disconnect USB serial converter (important as current leakage prevent power on reset)
- power off module
- jumper the main reset to ground
- power on the module
- connect USB serial converter or whatever you use
- startup serial terminal and get ready to press "4"
- release reset and immediately press 4
Can't promise that will work on your particular brand of module but does on most.
