Gigabit network switch won’t connect to our router unless connected to an 8 port 100mbps switch first - eviltoast

So basically our set up right now is Router/modem combo -> 8 port switch -> 24 port gigabit switch -> PCs in our bedrooms.

The location of our Router/Modem is in the floor below where each of the lan cables of our respective bedrooms are located. For some reason, whenever we try to directly connect the Gigabit network switch to the Router/Modem the port on the switch won’t light up signifying that theres no connection between them. However, when we connect the Router/Modem to the old 8 port switch first THEN connect it to the Gigabit switch, the port lights up and there’s internet. My dad and I have been at this for weeks and still couldn’t solve the issue (we aren’t exactly tech savvy lol).

From my understanding, all our cables are cat5e and the length from the router to the switch is less than 85m.

Any advice or opinions would be greatly appreciated