IPhone – How to fix “could not activate cellular data network” problem when returning to US after traveling and using a different SIM card

No - the sim card only provides an ICCID number to the phone. That number is read once per boot as the phone just uses the sim to identify what account to bill for service when registering with the nearby cell towers (IMEI and ICCID together let the system know what number to assign your phone and who to bill).

Something else must be causing your observed behavior.

Keep in mind, there is no guarantee two phones in the same location are not on distinct cell towers. Especially if one tower is overloaded or doesn't have high speed data so the phone /carrier might pick a faster connection that happens to have less bars of signal.

What does your carrier say if you call them for a site survey / diagnostic?

I have had exactly the same issues with my New Ipad 16Gb 4G. The only way I have found that really gets round it, is to manually select the Network provider, insteady of leaving it set to the 'Automatic' setting. Come on Optus sort this out !!