My son has out grown his bike so I’ve been looking at secondhand adds for a few weeks. Saw a Btwin Rockrider 500 for £250 and took a look. Like brand new and got it for £200. Only issue is the gear shifting is extremely stiff on both the front and back. Where should I start diagnosing the problem? Has SRAM combined gear leaver and brake leaver. Never had to deal with sram gear before and the online vids seem to point to cable damage. Appreciate any help.