You need to be a Creative Cloud subscriber to use LR Mobile (either the full package or the Photography package). Unfortunately, it's not included with the standard perpetual licence version hence your trial expiring.
Is there any way to upgrade my licence? The cost for the CC seems to be a bit high to only get the LR mobile since I already paid for the regular version...
No there is not, but LR 6 is probably just around the corner and if you sign up for the Photography Plan for $10/month then you’ll get LR5, now, and LR 6 when it is released and whatever releases after that as long as you keep paying. In other words you won’t have to pay the $80 upgrade for your current serial-number LR 5 to the serial-number LR 6 if you are a member of the CC version.
Obviously $10/month is $120/year and that is more than $80 once, for LR 6 upgrade from your LR 5, but you’re also getting LR Mobile and Photoshop for the $10/month.