Frequent disconnects - how to fix?

I am new to Openwrt and networking. I managed to install and do basic configuration of Openwrt on Archer A7. Everything is working fine except one thing. I am using a VPN on my laptop and it frequently disconnects and reconnets for a second or two. This happens something like every 10 or 15 min. When i was using stock router this was not the case, so the problem is in the router.

Is there any setting that i could change to fix or improve that?