How to recover iMessages sent to main phone number when using a different SIM card overseas?

Apple stores iMessages on their servers only until they are sent to your device. Therefore, they will be sent to your iPhone once you put your original SIM back in.

Another option would be to add an email address to your iMessage account and then even messages sent to your original number will be forwarded to your phone with the temporary SIM in if it has the email address associated with your iMessage account.

In my tests, sending an iMessage to a phone without SIM will result in the Not delivered status on the sender phone, and subsequent messages will be routed by SMS if the phone is configured to do so.

The receiving phone was connected via wifi to internet.

Putting the SIM back will not remediate this problem (probably because it appears as a failure on the sender's phone). Interestingly, the SMSs sent during the test did arrive after the SIM was inserted.

In order to be reached at your associated email address instead, the person messaging you will have to directly choose that address as the destination.

Here you can see a contact with two phone numbers and two email address but only two of them (one email and his mobile phone number) are iMessage destinations.

This makes everything more confusing because you'll end up with two (or more) different message threads for the same contact, with no easy way of sorting them out

—not impossible though, you can enter each thread and click though Details in order to establish the associated destination.

After further tests, I can't confirm the first part of my answer: I received messages connected to wifi without the SIM inserted. The only difference is that the person sending the messages had the Send as SMS option disabled.