Thanks for reaching out. The behavior you describe is fairly common among new SIMs from any carrier since initial network registration can take longer. This is only with new SIMs connecting for the first time.
As a carrier this is expected and we have no control over it. Unfortunately some hardware does not allow the proper amount of time for this initial registration. If this is the case with your hardware then doing an initial registration through other hardware such as a phone is good advice.
After this initial network registration you should not experience this again. This is probably why your first SIM is connecting without any issues.