How many routers can the Powerbank power and how long?
Considering an 80% efficient conversion to power up the routers, we have
$$ \frac{100 Wh \cdot0.80}{ 12 V \cdot1.5A /router} = \frac{80 Wh}{ 18W /router} = 6.66 h \cdot router.$$
Meaning for 1 router, 6.66h, for 2 routers 3.33, and so on.