I have a router connected locally to my office WAN, so router doesn’t have a public IP address, just a local one. Now I don’t have access to my Office WAN, so I created a VPN instead on my VPS so I can have a sort of PPP setup between the two and I will be able to connect to the router via my VPS.
So I basically setup a PPTP Client connection on my router to dial the VPN, which I thought will give my router a local address I can then connect with from VPS, but this isn’t the case. list | grep ppp on my VPS shows my router still has the public ip address of my WAN, though I can see 10.1.0.1 assigned to it via ip/addresses.
Is there a way to achieve this?
There should be addresses in the VPN connection (to be defined in /ppp profiles and/or secrets on your VPN server)
Since your office network is not aware of your address range at home, all replies from office nodes will be sent to their default gateway.
So you’ll need to masquerade your traffic and (and set up correct routes on both ends, of course).
Let’s assume your home network is 192.168.88.0/24 and your office network is 10.0.0.0/8
So on your VPN server:
/ip route
add distance=10 dst-address=10.0.0.0/8 gateway=<your-pptp-server-binding>
On your router in the office:
/ip route
add distance=10 dst-address=192.168.88.0/24 gateway=<pptp-out1>
/ip firewall nat
add action=masquerade src-address=192.168.88.0/24 dst-address=10.0.0.0/8 out-interface=ether1 #or whichever interface you plugged your router into
Just as a side note: If the office network is not yours and your boss doesn’t know about this, this will most likely be a reason to get fired.
…and I have the assumption that you mixed up WAN and LAN - your router is most likely connected to the office LAN 
-Chris