Q: Why has the status changed from 'External peer review in progress' to 'External peer review due' so many times?

This probably means that the editor is having a challenge finding peer reviewers for your manuscript. The peer reviewers must have accepted the initial invite, but later declined, perhaps due to non-availability or a conflict of interest. [This article provides an insight into why reviewers sometimes decline invites: Why peer reviewers refuse review requests]

Sure, you can email the journal about this. You can also ask them how much longer it will probably take to find the peer reviewers. In case you know of any reviewers and the journal allows you to suggest some, you may do this as well. Based on the journal’s response, you may decide your next steps. For help with mailing the journal, if needed, you may refer to this handbook: A practical handbook of templates for communicating with the journal [The handbook is available from a sister brand, R Upskill, and includes email templates for communicating with the journal in a variety of scenarios.]

All the best for mailing the journal, and then, hopefully, soon seeing progress in the review!