You're not going to get that fill port off without trashing the radiator. It's likely glued or soldered and if you twist it too hard I'd put money on the end tank cracking before the filler cap moves. Don't search for YT videos of people refilling a Nepton 240M, look for videos of people replacing the tubing on AIOs. You'll get more examples and tips that way.
The only way to refill most of these AIOs is to remove tubing (it looks like you should be able to cut the existing hose off) and replace it with the standard clear tubing and hose clamps as used in most custom loops. Not only will you be able to see bubbles, you'll be able to tell when it's time for new coolant. As for tubing diameter, you'll probably want to cut off the existing tubing first and measure the barbs on the block and radiator - go for the closest ID tubing that is smaller than the barbs and use dish-soap to lubricate the new tubing so that it fits. If it were me, I'd add a T-piece and a valve or end cap to make refilling and bleeding it a 2 minute job (instead of a 2-hour job) the next time you need to do it.
You'll want to drain the system completely, flush it as much as you can in reverse to try and de-gunk it and then refill with anti-corrosion inhibitor and biocide because you'll be dealing with mixed metals in an AIO. Too much anti-corrosion and your system will gunk up, too little and it'll corrode - so follow the instructions on the bottle carefully!
https://www.aquatuning.co.uk/water-...8/phobya-2-way-ball-valve-g1/4-knurled?c=2672 - an example of what I'd use as a fill port off a T-piece.