Can you try using the same SIM in a different module/modem ?
Taking a long time to register is usually a network issue - the network taking a long time to “authenticate” the SIM.
This can especially be the case when first registering a new SIM, and/or when roaming.
Probably (one of) the most frequent flaws with “own design” PCBs is the power supply:
GSM modules (in fact, radio transmitters in general) take very large peak currents - so it needs very careful PCB design to ensure that the power lines remain clean & stable.
etc, …
This usually manifests as as comms not working at all but, I guess, if your design is “marginal” it could be “struggling” but getting there “eventually”…