Part of the Furniture
Can you try setting the metric (hop value )to 1 on the main router.Thanks @Martineau .
Yes, this is the static route on my main router (AX86)
View attachment 31502
Yes, there is a reserved IP (192.168.1.98 as shown in one of previous post) for AC87U which is the downstream router, and yes it gets an automatic WAN-IP (192.168.1.98) from the main router (BTW, just for testing I changed that to static IP but it does not change anything).
So my conclusion is that it is a yes on all your 3 points.
There is definitely something that prevent/block traffic from 192.168.1.x devices to 192.168.98.x devices.
Hopefully it will then show the static route(s) for the downstream router(s)...in my case
ip route | grep metric 192.168.50.0/24 via 10.88.8.251 dev br0 metric 2 192.168.1.0/24 via 10.88.8.251 dev br0 metric 2
then query on the main router the route to a device attached to the downstream router...in my case
ip route get 192.168.1.111 192.168.1.111 via 10.88.8.251 dev br0 src 10.88.8.1 cache mtu 1500 advmss 1460 hoplimit 64