I am using two routers, one for connecting through VPN, and the other for internal network. My issue is after I connect via VPN, I can't access the internal network in other router. Here's my setup:

DrayTek Vigor 2832n
Used to connect to outside network via ADSL, and also used to connect to my network via VPN.

Firmware: 3.9.1 (latest)
Router IP (LAN1): 192.168.0.1
Subnet (LAN1): 255.255.255.0/24
Bind IP (LAN1): 192.168.0.5 (for the other internal router)
DHCP (LAN1): enabled starting at 192.168.0.2
Static route (LAN1): no rules
VPN IP (LAN1): static at 192.168.0.4

TPLink WR740N
Takes an ethernet connection from DrayTek router, where it is input in the WAN port.

Firmware: dd-wrt v24-sp2 (latest)
WAN IP: 192.168.0.5
Router IP: 192.168.1.1
Subnet: 255.255.255.0
DHCP enabled starting at 192.168.1.100

I have several laptops connected to the TP Link router, where IPs are in the range of 192.168.1.x and through any of then, I can access the router webpages for both TPLink and DrayTek routers.

In addition, I can successfully VPN to my site from outside, where I can open the DrayTek router webpage. However in doing this, I am neither able to open the TPLink webpage, nor open any PC connected at the internal 192.168.1.x network.

Here's the ping status in this scenario:

192.168.0.1 (DrayTek: Success)
192.168.0.5 (TPLink WAN: Success)
192.168.1.1 (TPLink IP: Fail)
192.168.1.100 (PC on TPLink network: Fail)

Here's what trace route shows:

192.168.0.5: 192.168.0.1 to 192.168.0.5
192.168.1.1: 192.168.0.1 to 185.17.235.2xx to 185.17.235.3x (no idea what these 185.x IPs are)

I thought its an issue related to the need for a static route in DrayTek router, but when I create one I get "Status: Invalid". Here are my settings:

Destination IP: 192.168.0.5
Subnet Mask: 255.255.255.0/24
Gateway IP: 192.168.0.1
Network Interface: LAN1

Hence, I am not sure what is wrong with these settings.

Also, I originally only had DrayTek router by itself, but weirdly enough, clients kept disconnecting ethernet connections every once and a while, which was causing issues in my network. I never figured out the reason, and since I need VPN access, I put this setup together.

Presumably the TP-link router is "NAT"'ing the data from its WAN port onto its LAN ports, rather than Routing?

If so, it will treat the Draytek, just like it would normally treat the Internet ... it does address translation, so the Draytek only ever sees one IP address (that set on the TP-Link WAN port), and it (the TP-Link) doesn't allow anything inbound. (Unless "Open Ports", "Port mapping", "UPnP" etc are in play)