Unable to port forward 25565 even with my own router

I have a Asus RT-BE88U router along with my own modem a Hitron CODA56, and I used to be able to open a port on whatever I wanted with zero issue and recently maybe a month or two ago I suddenly can't do that anymore with no warning. I have no done anything different compared to when it was working to now, I am just confused as to why it suddenly doesn't work and if there is anything I can do to fix it.