Static routing between Huawei and TP-LINK not working

Question

I have a fiber internet hooked up to a Huawei EG8145V5 @ 192.168.100.1, DHCP turned on. I am sharing this connection with a second office that I added an ordinary TP-Link router connected via cable to the main Huawei router to extend the WiFi + create a new network, yes double NAT unfortunately but that's how they wanted, to have a whole set of ip addresses, 192.168.0.x, separate from 192.168.100.x.

Everything works fine for their needs except for one edge case, one of the staff happens to work in both offices and while at the main office he needs to access a resource in the second office.

From the second office I can ping and access resources on the main network, 192.168.100.x. But from the main office I cannot ping or access network resources on the secondary router, 192.168.0.x.

I logged on the Huawei and added a static route but that didn't work, can't ping or access any resource on the secondary router, the TP-LINK with DHCP server turned on, 192.168.0.x, but I can ping the other way around, what can I do in this case to fix this since they insist on having different IP addresses on the second router? I though static routing would fix it, btw on the TP-LINK I disable all the firewall features.

Features requested is accessing network share on the TP-LINK (windows network shared folder) from the main office and also the TP-LINK network has a firebird server (unblock in the firewall) which is necessary to access from the main office as well.

WORKS
192.168.0.0      192.168.100.0
TP-LINK      ->  HUWAEI

for example: from a PC 192.168.0.2 I can access the main router @ 192.168.100.1 or a network share @ \\192.168.100.15 without no problem.

DOES NOT WORK
-----------------------------------
192.168.0.0      192.168.100.0
TP-LINK      <-  HUWAEI

for example: from a PC 192.168.100.15 I CANNOT access the second router @ 192.168.0.1 or a network share \\192.168.0.2.

Main Office Huawei - Static Route

Main Office Huawei - Static Routes

Main Office Huawei - Static Route With Gateway

Main Office Huawei - Static Routes With Gateway (when I add the gateway to the static route it doesn't show up on this table)

Answer 1

create a new network, yes double NAT unfortunately but that's how they wanted, to have a whole set of ip addresses, 192.168.0.x, separate from 192.168.100.x.

A separate subnet does not require double NAT at all. It will be possible to disable NAT on the TP-Link as soon as you get routing working.

when I add the gateway to the static route it doesn't show up on this table

If the destination of your route is 192.168.1.0/24, it does not make sense to specify an 192.168.1.x gateway address because the router doesn't know how to get to it yet. So the router's firmware just refuses to add such a route.

The gateway address must be something the router already knows how to reach directly – i.e. it needs to be an 192.168.100.x address in your case (the TP-Link's address that is "facing the Huawei router").

A route without a gateway will not work because the destination network isn't actually on br0 directly, it's behind another router.