A few ADSL modem/routers have the capability of separating the modem section from the router section, and offer a WAN port (usually by re-purposing LAN port #1). But the vast majority of ADSL modem/routers hard-wire the WAN-side of the router to the downstream-side of the ADSL modem. The TP-Link unit that you have seems to be of the latter hard-wired type, and therefore unusable in your new setup as a router.
If you disable the router features in the TP-Link (e.g. DHCP server, NAT, firewall etc.), then the LAN ports could be used as a switch and the wireless section used as an access point. Such a setup requires that the BSNL WiMax unit be the router for your network, and provide DHCP and NAT services. As you planned, one of the TP-Link's LAN ports would be connected to the BSNL's ports.
There are three interfaces that you have to setup in the TP-Link: Internet, LAN and Wireless.
Try to setup the "Internet" (WAN and ADSL side) interface to benign values, e.g. try setting the "Virtual Circuit - status" to "disabled".
You should setup the TP-Link's LAN interface with a static IP address that is within your subnet but outside the assignable DHCP range (issued by the BSNL WiMax). For instance my modem/router has IP address 192.168.1.1 for its LAN side, and its DHCP server hands out IP addresses in the range 192.168.1.100 through 192.168.1.199. A second wireless router (used in the same manner that you want to use your TP-Link) has (static) IP address 192.168.1.2 for its LAN side, and all of its "router" features are disabled.
The Wireless interface should be an ordinary WPA configuration.
There is no need to change the modem to 'router' mode.
You've described this topology:
===coax=== <WAN>
cable modem
<LAN> -------[ethernet switch]-------- <WAN>
synology router
<LAN> --- etc.
In this situation, it doesn't matter whether devices connected directly to cable modem's switch will get public or private addresses, because you wouldn't want to have any devices connected there at all:
Your main reason for using the Synology router was its protection features, and connecting devices to the cable modem's switch would completely bypass the attic router.
From the attic router's perspective, the switch is on the "WAN" side. It is a separate network from your main LAN, it has separate addressing, and the Synology router does not serve DHCP on that interface. (Even if you statically configured the addresses to match Synology LAN, it wouldn't work as the devices would still be physically in the wrong network.)
Devices connected to this switch wouldn't be able to access your main Synology LAN, unless you configured a hole through its firewalls and security features. (Or you could use port forwarding to get into the LAN, but a home network that relies on port-forwarding inside the network is just not good design at all.)
Your main LAN would be able to access devices on this switch (as long as both routers use different subnet prefixes), but the Synology router would perform NAT on the connections, meaning these devices wouldn't see the real client IP addresses. This is just a minor issue (and in some routers a NAT exception can be configured), but it can be an annoyance depending on what exactly you run in the basement, and again – not great design.
If you need the switch because you must have Ethernet ports available in the basement, there's only one option: basement → attic → basement. Run one Ethernet cable from the modem directly to the router's WAN interface, and a second cable back from the router's LAN to the basement switch.
Once that's done, there is absolutely no advantage in having the modem in 'router' mode.
If you cannot run a second cable all the way, the same can be achieved using two managed switches with VLAN tagging:
- On both switches, configure 1) one port in 'access' mode for the "WAN" vlan, 2) one port in 'trunk' or 'all VLANs tagged' mode; 3) the remaining ports in 'access' mode for the "LAN" vlan.
- Install one switch in the basement, the other in the attic.
- Connect the switches together using the "trunk" port.
- On the attic switch, connect the designated "WAN" port to the router's WAN side, and one of the "LAN" ports to the router's LAN side.
- On the basement switch, connect the designated "WAN" port to the cable modem's Ethernet output. (Use the remaining ports for whatever you like.)
Best Answer
The answer is yes, sometimes.
Think of the fibre cable as a virtual ethernet cable - from what you have described (and it would be a common setup), thats how its behaving. The question is then "What is the Ethernet cable doing?"
It MIGHT be simply acting as an ethernet cable - in which case you simply need to clone the MAC address of the device its plugged into and you are on your way. This is unlikely though.
Arround my parts, PPPoE is used (PPP over Ethernet) - this requires a login and password, and possibly other tweeks - in my case, knowing what VLAN it was configured on - Once I had this information, I was able to program my Linux box to act as a router - ofcourse, my ISP was quite forthcoming with the details.
(Also, its unlikely you have a fibre modem - you probably find it is a media converter - ie all it does is convert Fibre into something that you can plug an ethernet cable in to - a modem is a very different beast)