watch unpaired, serial number unreadable, too small

Conflicting information! iPhone is OS 12.5.5, should be able to pair with watch v. 3, but when tried, says iPhone is out of date. Tried to call or chat but the brand-new watch's serial number is so small that even with headlight and magnifying glass, 2 tries returned not a valid S/N. Not paired so cannot get to settings site to get S/N. How to proceed? Salesman saw iPhone, noted OS, did not tell me of any issue for pairing. Not happy. Thanks for any help!