Can I replace an intel core I5-4300u (which Is already in the Toshiba Portege Z30 laptop) with an intel core i7-8750H and if so can the Intel core i7-8750H connect to both 2.4ghz and 5ghz WiFi?
No, no way. An upgrade processor must be socket, chipset, and Bios compatible. And, your existing processor cannot be soldered to the motherboard, which yours is, which makes the rest of this discussion moot.
You are mixing apples and oranges.
Your processor cannot be upgraded. If it were possible to upgrade the processor, you would have to ask Toshiba what processors are supported by their bios, and then select from those.
The wifi is a completely separate issue. It is typically a small card that can be replaced. However, you would need to ask Toshiba about compatibility.
I know you want to breath some extra life and performance into this old laptop, but you really should consider a new(er) laptop.