Use isp modem as access point

[justwantv]

Cliffs, want to still use my ISP modem as an access point but let my Opnsense box do everything else. Firewall, routing, dns, ad block etc.  Is that possible?

I have Opnsense installed as well as Sensei running fine.  I got a great deal on a sff Dell months back I needed a purpose for.

Sky lake i5 6500
16g Ddr3
500g old laptop drive (I was thinking less power consumption than a 3.5 but more reliable than SSD???)
4 port gigabit NIC

I recently had ATT gigabit fiber internet installed.  Best WiFi and internet we have ever had. Prior we had cable and two ASUS AC68u routers in mesh mode. The new Modem/router has the ONT built in.  It’s small, runs cool, has excellent coverage across my whole house yard etc and seems to have crazy fast speeds.  Nearly 900 mb/s on my iPhone with WiFi6. The only dig against the ATT equipment is of coarse lack of control offered with Opnsense.

-I don’t want to use the old routers.  They get so hot and seem like energy hogs and not really up to par as far as speeds and coverage go. 
-WiFi 6 APs are super expensive. I’m sure they will come down but for now they are out of reach.  We only have 3 WiFi 6 devices (So far) but all 3 are heavy use.
-Less devices plugged in means less power draw, less points of failure and just cleaner. 

So is it possible to use the ISPs modem for only modem and WiFi and still see all the wireless devices as if they were installed behind the physical Opnsense box. 

This is all just a hobby for me but like most here the family needs rock solid stability. 
Thanks

[ArminF]

Traffic as far i can follow looks like this

--->  WAN (ISP) Modem with WIFI ---> FW OPNsense ---> LAN
<-------------------------------------------FW OPNSense  <---- LAN

So your Modem would need to relay WiFI DHCP requests to the OPNsense FW while keeping the WAN ISP ip.
Then you could setup a Rule from WAN to your LAN for that range or better mac address devices.

I am not sure how "intelligent" your modem is. Sounds more like a router to me. You would need to check if it could DHCP relay or maybe route traffic somehow to your Firewall into the LAN if you have resources there.
All WiFI traffic needs to go to the OPNsense and this should route back through the modem to the internet.

What you can use is the DNS on OPNSense to use DNSBL to block some sites and addresses.
You can also allow the modems wifi range and setup static IP/DNS mappings for the clients on OPNSense.
The default route will always be the modem.
Somehow it should be possible but its not the usual way as WAN does not allow incomming traffic by default.