Why are all my ports blocked and why does port forwarding not work?

My internet still works, I posted this using the internet connection I'm troubleshooting.
This is what I have:
Modem: Arris DG1670A Bridged mode (routing disabled)
Router: Belkin router
Ethernet connection

What I'm trying to do:
Port forward: 25565 (for minecraft server)
I have also tried other ports like 25566 and 25564 with same results
I can join localhost (I can join my own server) but my friends can't. I'm giving them my external IP (not 192.168.x.x)

This is what I've done…
… On my router
-port forwarding (TCP & UDP)
-DMZ
-factory defaults

… On my modem
-port forwarding (TCP & UDP)
-DMZ
-Modem firewall OFF
-connected directly to modem

… On Windows 10
-Advanced firewall inbound and outbound rules for both TCP and UDP on 25565
-Turning Windows Firewall OFF
-Turning OFF all anti-virus/anti-malware including Windows Defender
-Allowed "Java(TM) Platform SE binary" through firewall (Private and Public)

… Called my ISP
-Said they don't block any ports and neither does the modem by default

… Tested my ports on different websites and programs saying they're blocked.

Weird things:
-I use Steam and tested all the ports here ---> https://support.steampowered.com/...-GLVN-8711 They're all apparently blocked but I'm still able to use all functions of Steam/play online games.
-Like the title says "all my ports blocked", I haven't been able to find a single port reported open. Even the common ones like 80, the list goes on.

Port forwarding on both "modem" and "router" really means you have two "routers". You have to forward on the first router (connected to the ISP) to the second router only…
Then, on the second router, you port forward to the hosting PC.

With port forwarding, DMZ has nothing to do with it. Either you do port forwarding or DMZ, you can't do both…