Theres APIs for setting that (Qt Location Open Street Map Plugin | Qt Location 5.15.5), however I am unsure what host to use. Currently trying to find one.
But seriously, this must be something like everyone how tries to compile the basic QML example is facing. How can this not already be discussed and solved like a thousand times? 
I suppose its not even SFOS but QT related, unless more recent versions of QT already ship with different hosts and work out of the box
[edit] And the API does not even seem to work. No matter what I set as "osm.mapping.host" (or osm.mapping.custom.host - the docs seem to be ambiguous here), I will always get errors complaining about otile1.mqcdn.com).