Hmm, sometimes the logs in luci can show some usefull information, you still got a dhcp lease from the sf1200 on wifi but no internet?, and on lan internet correct?, Or is it that over some time the dhcp fails and wont give any dhcp?, Or dns doesnt resolve?
I know from searching the forums there are 2 issues stubby sometimes stops which uses nextdns you can restart it manual, and I readed some where that the chatty ness of udhcpd with MSG_NO_ROUTE could cause troubles on other subnets see OP: Flint dhcp issue but this is only for openwrt chaos calmer 15.x I believe your router uses openwrt 18.6.x ?
And the wifi interface firewall zone points to wan or is it part of lan (thats fine to) ?, And you use no vpn or wireguard ?, This zone need to be forwarded to if you use a different network interface than the default ones.