Given that the people who wrote this page were also "maths geniuses" and say in step 10 that to calculate m you need to use trial and error, there's your answer. No-one can rearrange that formula to have a single m on the left-hand side.
Yes, an iteration to the solution is the only way.
For large values of 'm', the term (u^m / m!) could be approximated to something more tractable by using Stirling's formula for m! However, that would still leave the intractable problem of 'm' in the summation limit, and all those (u^k / k!) terms inside it where k is too small for Stirling's formula to be even approximately right.