Setup with one router in hq and several routers in offices

hi all,

i'm pretty new in openwrt world, but i want to learn.
i have the following setup with Mikrotik and it works, but i want to switch on openwrt:
4 locations connected to the internet, every location with public IP. every location has his own local DHCP. in HQ i have also 2 VLAN's, every VLAN with his DHCP and all 3 others should share these DHCP's located in HQ. with Mikrotik i've used EOIP and it works, but here i don't know with what type of tunneling and how to setup to works.

already tried with ipip tunnel and i can ping from one location the DHCP server from HQ, but it can't take IP from DHCP.

thanks and regards

You need to implement a Layer 2 tunneling protocol to get DHCP requests; or run a Layer 2 tunneling protocol over a Layer 3 tunnel.