I have an ftp server which I wanted to access from outside of my local network. It is located on the IP 192.168.0.23 port 21. I port forwarded port 21 and tried to connect with my phone while I wasn't connected to my wifi and it worked. The problem is I can't connect with any other device. Today I tried connecting with my phone again while I was away from home (so you don't think I was connected to wifi by acident) and it worked, but when my friend tried it didn't work. My network set up is: main ZTE router that is connected to the internet, TP-Link archer C7 which is connected to my ZTE router and I use it as an access point. My ftp server is also set up on Archer C7. I'm really confused with this problem because for some unknown reason it works for my phone but doesn't for everything else. Has anyone encountered anything like this?
P.S I contacted my ISP to ask them if they block my ports and they said they don't.