OK, so that's bad, obviously every device on the network needs its own unique IP.
You need to disconnect the AP from the network and connect a laptop or some other computer directly to the AP. Of course you need to power the AP with a power adapter or POE injector.
Login to it and change its IP settings.
Alternatively, you can disconnect the cable modem/router, so its IP does not exist on the network, connect to AP, change its address, reboot it, then reconnect the cable device.
All this assumes you are actually having the same IP for both devices on the internal network. The external IP will look the same for all the devices on your network, because the router is NAT-ing all the internal IPs to its external IP. You did not give us many details....