Correct configuration info

Continuing the discussion from GL-MT300N-V2 USB tether bridge to ethernet?:

Can someone point me the direction of the actual correct configuration to achieve this setup along with being able to use DDNS that my main router is providing? If only this little router had the range and the ports of it then i wouldnt need that one but until i can replace it i am stuck with this combo so any help would be greatly appreciated. Thank you