Thank you for your answer.
I have asked questions about where routers are deployed, since recently we become aware, that in some harsh environment (high humidity/vibration) situations intermetallic oxidation process between PCB contacts and SIM card can occur, which might prevent device from communicating with SIM card.
Because of that, when using or storing device in harsh environments, we would recommend to use specialized anti corrosion lubricant (we tested KONTAKT 61 from Kontakt Chemie) as well as cleaning the SIM contacts with isopropyl alcohol before the application of lubricant. Make sure not to touch the SIM contacts after cleaning as that might introduce corrosive oils from the hands which could compromise the effectives on the lubricant. That should allow to avoid or at least slow down oxidation process.
Even though from your current description is does not seem that devices are deployed in harsh environment, it might be good idea try to using anti corrosion lubricant and checking if that allows to avoid/resolve this issue.