hope all of you are fine...
My Question is: how to let the access point redirect the people to the main router when there's available place on the main router because I want to avoid the situation of not connecting to the main router while there's available slots and connect to access point which is suppose to be a support for main router when there's no slots for more clients? also how to let the main router redirect to the access point when the client cannot connect to it?
Note: I have TP-Link router and access point
I have tried several configurations using a second router (also TP-Link) as a second access point. From my experience so far, I do not think the conditioned redirect is possible with a simple router (may be dependent on the exact routers which you are using).
I think the situation is more likely an "either ... or ..." case.
Option A: You can disable DHCP of your access point and let your access point router redirect clients to your main router
Option B: You can open a subnet with your access point
Apparently, you would like to have all clients use the main router. Therefore, I would suggest option A, including changing the size of your main router's network dependent on the max client count you would expect.
The DHCP range of your main router will have to be changed. Dependent on your requirements, you may be able to achieve your goal here. Altering the range from /24 to /23 will already be a fair amount of new available IP addresses. If CIDR is not supported you may be able to change at least the last 16 bits: Your network will then be of the type:
192.168.xxx.yyy with Subnetmask 255.255.0.0/16
This might possibly be too much for a normal router but the max possible number of clients will not be reached anyway - so it might work just fine.