The IP address seems to be on the wrong subnet. My router is set up on 192.168.0.1 and the IP of the Pi is 192.168.0.2.
When I connect to the AP i get an address of 192.168.1.2 and the gateway is 192.168.1.1. So this is wrong for a start.
This is correct because the tether creates a NAT routed (not bridged) connection between the interfaces, so two different subnets will be used. There is no need to set iptables rules or enable forwarding as these are either handled within connman or already enabled in default config.
It would be useful to see "route | paste" before/after the tether is enabled, and "journalctl | paste" after a clean boot, enabling the tether, and connecting a client device. Just share the URLs generated.