Ch7465lg-lc from ISP and my TP-Link TL-WR841N/ND v10 Set Up

I have the ch7465lg-lc from ISP(UPC) and TP-Link TL-WR841N/ND v10 with OpenWrt 18.06.1 r7258-5eb055306f / LuCI openwrt-18.06 branch (git-18.228.31946-f64b152).
My setting is:
The ch7465lg-lc is the DHCP server an connected via LAN (Lan Port to Lan Port) to TP-Link TL-WR841N/ND.
Altogehter i have 3 WIFIs due to the Size of the flat:
2x2,4 Ghz (one on each Router, not the same name also different channel)
1x5 Ghz on the ch7465lg-lc for Movie Streaming
Everything is Running fine for 1,5 Days, then i have to restart everything, and i cant figure out way
I already Upgraded once from openwrt-17.xx to openwrt-18.06

1.)I didnt adjust anything with the IPv6, should I??
1.5.) If yes, some good instrucions?
2.)Maybe better some Overall instructions
3.) I could also provide the Log data...