Would this be a good use case for an SSR?

Sixfab uses the Telit ME910C1-WW chip on some of their USB models and the Quectel EC25-A for others. They are available as M2 form factor inserts, or you can buy just the chip.

Correct, it's a switching converter, the MAX17640. 5V is not powering up (getting ~2V out) and then the 3.3V LDO is not powering up.

I put an older variant daisy-chained board before and after the newer board. Both of the older ones come up, but the newer one doesn't.

I will check soldering disturbances and put my scope on the input line to see if there's a lot of noise. Thank you!

Thank you, I will reflow C4 and add C16, but C10 and C14 look fine from the sides.

The messy/missing joints near C17, C19, and C20 are optional components for the other side. (Unused surface mount USB port with through-hole mounting pins and an unused jumper.) Right-hand jumpers are all optional and unused from an earlier prototyping stage.

What do you mean by extra traces by the IC? And you're absolutely right about the small IC. It's the MAX17640, and at 2mm x 2mm it's a piece of work to land correctly.