Unknown surface coating
Very likely, the title of your question already hits the nail.

Since mobile phone tend to be "smart", filled to the rim with computing power and data, users wish to protect their companions with a coating that repells water.  Often, the devices are not only coated on the front side, which would at least hamper accumulation of residues of gliding fingers, but all around, including the back.  As a result, you might get a cell phone that may drop into a bowl of water, and still remain functional.

Among the different technologies available (a short presentation by CNN, back 2013 is found here), there are polymer coatings, or a mixture of anastase / $\ce{TiO2}$ and PFTE / teflon (like the coating of your frying pan) particles with some perfluorinated liquid (example).  For some, you need a larger installation that appears like a scilab, for others, you may prepare it at home with a spray can (example).  Hence the results in terms of performance, maintenance, and longevity may vary, too.  As a result, if applied both on front and back of the phone, not only water, but the components of "normal glue" (referring to what backs a velcro tape) may be repelled more or less efficiently, too.

Your observation that the coating appears little to non-affected both by water as well as iso-propanol (which for some coatings is a no-go, as it would tear the layer off), as well as for WD 40 (which may be summerised as a blend of apolar hydrocarbons (ref)) allows the speculation that the coating might be either polymer-based, or the triple mix of anastase + teflon + perfluorinated binder.  To discern between the two, remotely, is not possible.



If a mechanical, pocket-shaped mount like



(source)

or a pouch, to be bound around your upper arm while riding a bicycle, like



(source)

is not what you may deploy (or alter a little to fit your needs), and if the coating consists of the "polymer-only" variant, then maybe it is possible to establish a connection between the back plate of the phone, and either the velcro tape or something already connected to the velcro tape with the help of cyan-acrylate based gorrilla super glue.  These glues often bind very strong and to polymers, however partially dissole the polymer a little.  This chemical approach however comes with many "ifs" and uncertain outcome.