Check temps and clocks, the GPU might be working on low clocks and high temps (that model sadly has maybe the best cooler, but badly designed as it runs very hot sadly), also it's hard to compare with the little info you gave, I mean I can use a even lower powerful GPU than RX480 and have much better FPS by just playing on lower settings and lower resolution...
Are you really talking about same exact resolution and visual settings in the same games from the laptop to the RX480?
Also that CPU (actually APU) is very poor in gaming performance and check if your not using it's actually built-in GPU instead of the RX480 and my advice is also to try windows performance mode on the power settings and OC that CPU the max you can (if you know how to do it safely) and make sure the games are using the RX480 and not the built in Radeon R7 IGPU...
Anyway sadly you will never be able to get the full RX480 performance with that ~60€ CPU with a ~300+€ GPU back when it released...
ps: Also If I were you I'd flash that Asus Strix RX480 to Asus Strix RX580 like I did (Warning: do at your own risk, no one will be responsible if any problem happens and warranty won't cover it).
