Your U11 Plus features a microSD memory card slot. If your personal computer is compatible with that type of card (Most laptops are so-equipped), you should be able to save a music file (The HTC U11+ supports these formats: Hi-Res Audio certified) from your computer to a microSD memory card, and then, from the memory card to your phone. Your manual should tell you how to assign the file as your phone's main ringtone.
The HTC U11+ is an Android phone and there are plenty of free ringtone apps such as MP3 Cutter, RingDroid (open source) and more.
Here are some of the free ringtones (HTC U11+-compatible) offered here at MartinsRingtones.com:
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