Given your concern for the lack of a firewall, I can only assume you have the WAN of the ASUS connected to a LAN port on the primary router (altice, of which I know nothing about). IOW, your ASUS is daisy-chained (WAN to LAN) to the primary router, creating a double NAT configuration, where your ASUS manages its own IP network, DHCP server, firewall, etc., separate from the altic.
The reason I'm going into detail here is because you *could* extend the reach of the ASUS if it was *bridged* to the primary router over wireless using the repeater. But by definition, a repeater means the primary router manages the IP network, DHCP server, firewall, etc. IOW, that's the whole point of using a repeater; to extend the reach of the primary router!
Even so, one problem I see when looking at the manual for your ASUS is that it seems to rely only on WDS rather than standard wifi protocols. WDS is proprietary and so incompatibility w/ other WDS enabled devices (assuming your primary router even supports WDS) is VERY common. Most of the time, it's only going to work w/ very similar hardware/firmware between the two devices.
What you might want to consider is perhaps getting yourself a wireless ethernet bridge. You configure it like any other wireless device by connecting it to the primary router's AP, and patch the ethernet side to the WAN of your ASUS. In effect, what you'd done is created a *virtual* wired connection from the ASUS to the primary router. But in all other respects, nothing else is changed.