Is there a way i can figure out what the problem is?
Have you seen this thread https://community.bt.com/t5/Home-setup-Wi-Fi-network/Port-Forwarding-BT-Smart-Hub/m-p/2068801#M17754...
I have let Java and Minecraft and port 25565 through on both TCP and UDP, and also made an exception in Windows Defender for all of my Minecraft folders and my server folder, and my friend still cannot join.
Perhaps the fault's at your friends end, can anyone else join?
No, no one else can join