Since nobody seems to have seen this with an immediate idea in mind (and not everybody uses this Google backup capability) …
As this backup and restore method is endorsed by Fairphone, you could contact Fairphone support about it … https://support.fairphone.com/hc/en-us.
Forum consensus is that calling them should give you a way higher chance of fast communication or results.
Alternatively, if you happen to have #fairphoneangels in your vicinity (see #angelsmap) with Fairphone 2 or Google backup expertise, you could ask them whether they would have some idea or whether they would be able to take a look in person.