Replace 173 Wh lead acid battery with 20000 mAh power bank
UPS usually uses a 12 volt lead acid battery.  The site you linked to says the UPS uses a lead acid battery.  It doesn't say it's 12V, but they usually are.

It is specified as 173 watt hours. That's something like 14 amperes at 12V for one hour.

Your power bank is 20 ampere hours.  Giving it the benefit of the doubt, that would be 100 ampere hours (5V at 20 A for one hour.)  More likely, it's 74 watt hours (3.7V at 20A for one hour.)

That's quite a bit less than the original battery.

The UPS expects 12V from the battery.  You'd have to use a boost converter to go from 5V to 12V.  You'll lose power that way, and runtime.

It is also unlikely that the powerbank could deliver the needed current.  It would take more than twice as much current from the power bank to boost it from 5V to 12V.  It is unlikely that the power bank can supply the needed current.  Power banks usually have USB connectors.  I'm pretty sure the USB connector itself would start melting from the high current if the power bank would deliver it.

As a practical thing, you cannot use your powerbank in your UPS. 



Look at it this way:

To get 230VAC from 5V, you have boost the voltage by a factor of 46.

To get the required current (5A) at 230VAC, you have to put in 46 times as much current on the low side.

Your power bank would have to deliver 230 amperes at 5V to the boost converter to get the needed 12V.