Regular 220V UPS is not efficient especially if u using Mikrotik or UBQ equipment - 2 battery UPS convert 24V in to 220V than we use adepter converting 220V in to 24V we have massive losses
Someone may have already done it but let me show how to
What u need is
Old non working 2 battery UPS, meanwell AD-55B -this come with build in UPS function
and some cables as shown on the picture
remove everything inside
By the way u can reuse some of the cables - if not u can make new one
Now check if the batteries are ok , but probably they are death
next
next
Place AD-55B as on picture connect all the cables
Put PoE splitter outside as u wish
now if the old batteries are death, u have to bay new one I suggest u to use 12V 7.2A they are about 8 Euros.
Connect the batteries
Place them inside
And at last there u have it
working 24V UPS 
at last place careful UPS cover
now lets do calculation
we have 24V 7.2A system, we can use only 50% of battery that is about 3.1A
we have 24x3.1=74.4W
If we have 3 Units Omnitik(max pow. cons=7W) and 2xSXT(max pow. cons=7W)
3units x 7W=21W per hour 74.4/24=approx 3.5 working hours - but that is never going to happen 
Omnitik and SXT are using more or less 4W (thats average consumption) 3x4=12W average consumption
74.4/12=u have approx 6.2 working hours maybe more
Cost:
Since we use old UPS we can reuse some of the cables inside we dont have to spend
AD-55B - 21 Euro
2x7.2Ah Battery =2x8=16 Euro
cables and PoE splitter = 3 Euro
Total= 40 Euro - As cheap as baying low cost Chinese (probably will last 6 mounts) UPS
Very good also I would suggest using polyswitch resettable fuses used on each RJ45 POE sockets, this will
isolate power faults on one device effecting another device.