The Simple Answer
DC Power is defined in terms of W = 1 V * 1 A - that is, the power that is delivered by sustaining 1V potential with 1A of current.
Thus, a battery pack that can deliver 5400mAh, that is 5.4Ah, while sustaining voltage of 10.4V (this happens to be running in my laptop right now), can in theory deliver up to 5.4 * 10.4 = 56.16 Wh = 56160mWh.
The Complicated Answer
The above get a lot more complicated with different battery chemistries, and with different measurement methods. Firstly, the mAh rating can depend on the actual current draw - in general, the more current you draw, the less capacity the battery has, but there are exceptions at both ends of this guideline (if you draw too slowly, self discharge affects your measurement, and if you drive quickly enough, the battery gets warmer, and if it doesn't break, it tends to perform better).
Also, the voltage across the battery changes with the load - this is at least simple, the more current you draw, the lower the voltage across the terminals (this is due to internal resistance).
Finally, some devices are essentially dumb loads (battery powered tools), and draw as much as they can from the battery.. and some devices handle voltage and current changes in a more intelligent manner (mostly laptops and other DC/DC converters).
This means that for dumb loads, you are more concerned with mAh ratings (perhaps measured until battery voltage remains above some usable threshold).. since this can be used to calculate time-to-empty (which is really what you or your users are after), and dumb loads are approximately constant current/constant resistance loads.
For smart loads, the discharge controller (DC/DC converter) would actually try to drain constant power - the lower the voltage, the more current it drains so that it can continue outputting constant power on its business end.
The best system is to simply have the charger constantly powering the battery. And the devices constantly drawing power from the battery at the same time. This is equivalent to a "full online" UPS. As there is zero transfer time. (cheap mains UPSes have a small drop in the mains output when they transfer the load from mains to battery during a power failure).
Check the output from your charger as mentioned above. As lots of 12V battery chargers have poor output voltage regulation. And are typically designed to charge a battery, and then be disconnected. They will often overcharge the battery if left connected to a battery 24/7. Get a regulated power supply that you can adjust the output to 13.5V or whatever the battery manufacturer specifies as the ideal "float" voltage for that battery.
Also consider how reliable your SEPIC power supplies are. Especially in relation to over voltage protection on their outputs. You don't want a faulty SEPIC inverter to destroy your connected devices from over voltage. Do a search for "shunt regulator" and "crowbar circuit" for examples of over voltage protection circuits.
And make sure that you put some fuses on the connections to your battery, as close to the positive terminal as practical. As lead acid batteries can provide very large currents if a short circuit occurs. The main purpose of the fuses in this case, is to stop a fire from occouring due to overloaded wires, if one of the SEPIC inverters or your battery charger fails short circuit.
Best Answer
To make an "apples to apples" comparison, you'd need the capacity rating for both devices in watt hours.
What you have is VA for one device and mAh for the other.
VA (volt amperes) and watts are a power rating.
mAh (milliampere hours) is (sort of) an energy rating.
The unit "watt" doesn't have time factored into it. Energy always has time factored into it.
You can't compare the two ratings without more information than you've given.
A UPS will often have a power rating and a time rating (600 VA for 10 minutes, for example.)
The powerbank rating is usually the capacity of the battery rather than a rating that directly tells you how much energy you can get out of it for how long.
Energy is the product of voltage, current, and time.
Your power bank gives you the product of current and time for the battery discharge, but doesn't give you the voltage of the battery.
The UPS gives you the product of voltage and current for the output, but no time.
You cannot calculate the available energy from either one, and are missing different bits of information you would need.
You can't really make a valid comparison of the two from the information you are given. It's like comparing an apple core and an orange skin. Does not compute.
I'd find a UPS that gives you the VA rating and a runtime. From that you can estimate how long your devices will run.
Multiply VA by the time in minutes. Call that E.
Multiply the volts and current for each device to get a VA rating for it. Add up the VA ratings for all of your devices. Call that P.
E divided by P will give you an estimated runtime in minutes that you can expect from the UPS. It will be somewhat pessimistic because the VA ratings for the devices will be the maximum rather than average values.