HTC U Ultra: Can I use dual SIM on this device?
Enter download mode by powering down the device and then holding both VOL_DOWN and POWER to power it back up. It will boot to a screen saying hTC download mode.

The 3rd line of the displayed info will say something like htc_oceuhl PVT S-ON. uhl corresponds to the single-SIM variant, dugl is the international dual-SIM variant, and dtwl is the Chinese dual-SIM variant.

You can exit the mode by selecting "reboot" in the download mode menu.