Changment of harware

Hello everyone,

I am kind of a newbie in the field of networks and I have a theoretical question for you. I tried yesterday to change from my current mikrotik router to a hardware firewall + wifi hotspot. The only way to connect this hotspot to internet is the following : modem ------ mikrotik router ------- firewall -------- wifi hotspot.

But I would like to remove the mikrotik router from this installation (because this way I have two different wifi network…). So there are my questions : is it possible ? why does’t it work when I connect the firewall directly with the modem ?

I hope it is clear enough,
Thank you in advance