The router freezes during file transfer (youhua wr1200js)

I have a computer and a NAS.
When I copy files via SMB to the NAS, only a few GB are copied (always a different amount) and then the router "hangs".
No other device on the network has access to the router (such as luci) and the Internet. I always have to turn it off and on.
I use OpenWrt 19.07.0 r10860-a3ffeb413b / LuCI openwrt-19.07 branch git-20.006.26738-35aa527 with kernel version 4.14.162 and router youhua wr1200js with architecture MediaTek MT7621 ver:1 eco:3.

I think the roter freezes even when there is normal traffic on it (watching videos, visiting sites, etc.)

Can you please advise me?

Hi with openwrt 19.07.5 I have disabled Software flow offloading (firewall)
and no longer had a freezer

Hi, I too have the problem with my NAS hanging on large file transfers and am also running 19.07.5 but am unable to find the software flow offloading setting in the Network / firewall section.

What am I missing?