UART on the HAT UP4000

I would like to use the UART on the HAT connector. I am having trouble getting a basic loopback test to work. I am using Ubuntu Server 22.04.

ls /sys/bus/pci/devices/0000:00:18.?/dw-apb-uart.*/tty/ | grep tty
/sys/bus/pci/devices/0000:00:18.0/dw-apb-uart.3/tty/:
ttyS4
/sys/bus/pci/devices/0000:00:18.1/dw-apb-uart.4/tty/:
ttyS5

Judging by the wiki, the HAT UART should be ttyS5, so to loopback I short the Tx and Rx and run:

sudo screen /dev/ttyS5 115200

But I get no returned chars. I tried on tty4 to be sure but again, no chars.

Further information on my system. ttyAMC0 and ttyUSB0 are known attached devices, they work OK:

sudo dmesg | grep tty
[ 0.132638] printk: console [tty0] enabled
[ 8.074938] cdc_acm 1-6:1.0: ttyACM0: USB ACM device
[ 8.094024] dw-apb-uart.3: ttyS4 at MMIO 0x91224000 (irq = 4, base_baud = 115200) is a 16550A
[ 8.124707] dw-apb-uart.4: ttyS5 at MMIO 0x91222000 (irq = 5, base_baud = 115200) is a 16550A
[ 8.140810] usb 1-5: FTDI USB Serial Device converter now attached to ttyUSB0

Hoping somebody can offer some advice on what else I can check, am I right in thinking ttyS5 is my port?