Firmware: Apple Time Capsule Custom

There is no "one-click" custom firmware (like DD-WRT) for the Time Capsule due to its encrypted and signed bootloader.

This gives you a full Linux server with working 5GHz Wi-Fi (using the proprietary broadcom-sta driver). You can then install: apple time capsule custom firmware

If you’re ready for that trade-off, read on. There is no "one-click" custom firmware (like DD-WRT)