Each EG4 6500EX inverter ships with a serial RJ45-to-DP9 cable (see:
https://diysolarforum.com/threads/eg4-6500ex-cant-update-firmware.60715/post-751581). To connect this to the PC you need to buy a serial DP9-to-USB adapter cable which has to be connected to the provided cable (e.g.
https://www.amazon.com/gp/product/B000A1KD1K). I've successfully used this to update the firmware on my inverters.
I've never used the special cable from Signature Solar which you've mentioned (
https://signaturesolar.com/eg4-firmware-cable-for-6kw-6-5kw/). But it sounds like that this may be a replacement for the above described solution with the 2 cables - but I'm not sure.
Don't use the RS485 cable which Signature Solar usually includes with an EG4 battery order (the one with the 2 cables attached with screws to the USB adapter)! This is only for upgrading the firmware for the EG4 batteries (see the first link in my answer).