My modem (A) is connected to internet ISP. Doesn’t have strong Wifi support, so a router (B) is connected to modem (A) to provide Wifi support to the whole house.
I can login in the modem (A) GUI interface via 192.168.1.1.
I can login in the router (B) GUI interface via 192.168.0.1.
I have a Linux notebook connected over a ethernet cable to my modem (A).
My android phone is connected to the wifi router (B).
Firewall (ufw) is well configured on my Linux notebook.
The problem is, I only can connect Syncthing between my notebook and android if both of them are connected to Wifi on the router (B).
If notebook is connected to modem (A) via an ethernet cable and android to router (B) via Wifi, the devices can’t reach each other!
UPnP is disabled on both (A) and (B) for security reasons (?).
Modem (A) model is a Technicolor ADSL Router and Router (B) a TP-Link.
So, how can I connect both of them on this setting?
Modem and router have different networks, so they (normally) cannot talk to each other.
Syncthing finds other devices and there IP addresses by broadcast/multicast, which only works inside the same network.
You can either enter the IP address of the laptop into the addresses field of the remote device setting on your android, or you can connect your LAN to the router instead of the modem, so every device in your home (LAN or Wifi) is in the same network.
My local is 192.168.1.104 (Static notebook) and android (DHCP) is 192.168.0.102.
or you can connect your LAN to the router instead of the modem, so every device in your home (LAN or Wifi) is in the same network.
Does that mean connecting the notebook (Linux) to the Wifi network? (Sorry I don’t understand well networking). If so, yes, Syncthing works normally doing that.
If you change dynamic to tcp://192.168.1.104:22000 in the device dialog for your laptop on your android, it should be able to connect. But only if both devices are at home.
You router should have additional LAN ports, so you can connect your laptop and everything else with a cable (not Wifi) to your router too, so that all devices are in the same network.
you could also set your router (B) to act as a simple access point (disable DHCP, set static IP within network (A) and plug cable into LAN port instead of WAN)
You can also use both, i.e. tcp://192.168.1.104:22000,dynamic, such that it both tries discovered addresses and this static address. Thus it should also work when outside this network.
Generally you should heed the networking advice given and acquaint yourself with the settings or your router/modem to achieve a single network (and then transmit multicasts between wifi and lan, which is often prevented by default). Having generally the same “base-setup”, I set the modem to bridge mode and connect everything to the router, thus you need to deal with one device only to get your internal network running the way you want to. If your router has no LAN ports, then my approach wont work, but the one suggested by @Alex will still work.