On MT300N-V2, you missed open port, the firewall configuration as below:
config redirect
option target 'DNAT'
option src 'wan'
option dest 'lan'
option proto 'tcp udp'
option dest_port '80'
option name 'wan_lan'
option dest_ip '172.16.0.1'
option src_dport '80'
config rule
option name 'Allow-web'
option dest_port '80'
option proto 'tcp udp'
option src 'wan'
option target 'ACCEPT'
config redirect
option target 'DNAT'
option src 'VPN_client'
option dest 'lan'
option proto 'tcp udp'
option dest_port '80'
option name 'vpn_lan'
option dest_ip '172.16.0.1'
option src_dport '80'
Besides, the 80 port should be opened on your LAN device.