dedicated vpn router behind modem/router

I have from ISP ZTE ZXHN H267N V1.0.,as modem/router.
I bought hap mini mikrotik router(RouterOS) to be dadicated vpn router.
What i wanna do is use ZTE as modem/router connect throught ethernet hap mini and evry device I connect to hap mini to go through my VPN provider(I want to set up OPENVPN or at least L2TP/IPsec).
I have tried many guides and always something was wrong and it didnt worked.Even when evrything looked fine when I tried,‘what is my ip’ it shows my normal IP.
Any help to achiev that would be welcome even if someone can point to proper guides.
PS. I want my second router(hap mini mikrotik to connect to my VPN provider and evry devices connects to it to go through VPN).
If anyone can help it would be apreciated cause I am failing 2 weeks now.