Have you tried using the AT commands?
ATI5 shows all the parameters and among them are the MIN and MAX frequencies.
Using the ATSn=x command you can change the radio parameter n to value x. Then use AT&W to write the changes to memory. More detail here
Was wondering how well that would work, I expected the antenna matching circuit to be design for the specific frequency. Maybe 868MHz and 915MHz are close enough and the loss minimal.
868MHz are allowed in the EU but it’s the conditions which goes with it which makes it more tricky. Take a look at ETSI EN 300 220-1.
To be able to legally use it you have to turn the power down a lot (like you have already done) and conform to some polite spectrum access techniques (like LBT or limited duty). On top of that you have to stay within the allowed channel bandwidth which forces you to drop the air data rate a lot.