1. The network is unstable because wds is enabled on your primary router. Router bridging only requires the secondary router to have wds function. Turn on wds on the secondary router to bridge the primary router (remember to change ip and turn off dhcp), and the bridge will be displayed successfully. Note: The main router must not open wds, otherwise the network must be unstable.
2. Wired Internet access is not possible because the ip and gateway Settings of the device you connect are incorrect. According to the description of your problem, connect the network cable from the lan port of the secondary router to the device, you can manually set the device ip to 192.168.1.100-199, as long as it is between 100-199, of course, do not conflict with the ip of other devices. The setting of the gateway is the most important. The gateway of the device must be set to the ip address of the primary router, that is, 192.168.1.1. dns is different from place to place, just fill in the query.
3. I believe you do as I said, you can connect the network cable from the secondary router!