Access to HA from the net trough 2 routers

Installation
1

I have a little problem, i i hope somebody can Help.

I explain the history:
I had a working router with a lot of devices after it.
The router is a asus rt-ac68u, which is also the DHCP server
The i get a new internet provider who deliverd a new glass router, a fritzbox 5490.
The fritzbox is in front of the asus, so everthing keeps on working.

My homeassistant is behind the Asus router with ip address: 192.168.1.300
the asus router has the ip adress from the fritzbox (192.168.178.200)

How can i set up acces to homeassistant from the internet?
Do i have to forward a port? and on which router i do have to do that?

2

Are you able to put the Fritzbox in bridge mode so that it only acts as a modem?

3

Somebody told me that is is not possible to put the fritzbox in bridge modus.

4

bridge mode is not possible, but i found out that there is an other option to give a device full acces tot internet.
I did this thing, and now it is possible to access HA from internet.

Thanks👍
Without your answer i didn’t found out this option.