WD MBL 3TB connection issues

The 2 networks is in place just purely because of my Sky Modem - If I try to merge the pair, the Sky Modem has a fit of giggles and bricks.

The reason I’ve connected the MBL to the Sky Modem is to remove complications I had when I had it connected to my internal network… I ran through firewall issues, I setup the firewall rules on the Sky modem too - But I still couldn’t access it from outside my house.

But now I’ve connected it to the sky modem, I can’t access it in the house! lol… Oh it’s a headache on a stinking hot day.

Just thinking - Maybe if I disable the Sky modem’s firewall completely and just turn it into a modem, then connect the MBL to the router again and setup the firewall rules- Surely this would mean it should run both indoors and out, as the Sky modem will no longer be able to intervene with what the routers firewall is allowing and blocking?

I’m not sure if I’m over thinking this and can’t see what’s basically obvious but it’s definitely annoying lol.