The Intel Core i5-8400 doesn't have the K suffix in its name, which indicates an unlocked multiplier. That means that you can't overclock it easily.
One option is to increase the BCLK, which can be raised to 102 MHz or 103 MHz from 100 MHz, but not more, for a 3% performance uplift, in theory.
The second option is to adjust the CPU boost settings, making it so that the highest boost state is enabled not just for single-threaded workloads, but all - no matter the core count.
When we tested the latter, we noticed that no matter what we did, the maximum frequency the CPU would run at was 3800 MHz, not 4000 MHz as you would expect by going with the maximum-rated boost frequency.
Whether this is a bug or a feature is unknown at this time.
Temperatures were of course not an issue and voltage changes were not needed, either, since the processor was always operating very close to its out-of-the-box power and heat levels.