Assuming there are no obvious large leaks, sounds like air is oozing out the sidewalls.
If it holds air for long enough to do so, go for a pedal. A 10 min ride should whizz the jizz round the inside of the tyre and seal all the gaps up.
If it won’t be rideable, brush soapy water over the outside, pump it up and check nothing is coming out at the bead. If that’s OK do the Stan’s Shake – basically hold the wheel horizontally and swirl it about so the latex coats the sidewall, flip it over and do same to other side, repeat until it stays up. You should see the air leaks as bubbles or white ooze on the outside of the tyre. If you can get it to a point it holds enough air to ride, then go for a 10 min spin and should sort it out.
(EDIT – if it’s “tubeless ready” – eg Bonty or Specialised 2bliss it will need sealant; normal UST ones don’t. IME once it’s set up it should be grand either way.)