Detect continuity in AC line during load shedding and take action via DC circuit?
You can try adding Schottky diodes on both input and output +V lines of your PB (6).







simulate this circuit &ndash; Schematic created using CircuitLab

D1, D2 should be Schottky. Not the best solution, but it might work as a "poor mans" backup power solution. Better solutions involve more complex schematics so I'm not sure if you can find parts for them in the current situation.

Or if your PB (6) supports both charging &amp; powering output at the same time you can disconnect your router (5) from DC-DC (3) and connect it only to the PB (6) output. That way it will work as a simple UPS. Some power banks can support this mode.