I'm not a pro in that domain. I am wondering what would be the best way to power in the office the RB5009UPr+S+IN so it can resist to power outage and go on the battery if it crashes. Any idea is welcome.

So the main connection right now is handled by a CCR22004 dual BGP homed that is plugged to two UPS (APC). This bring the connection to a CRS317 wich connect our local data pop (4 servers). One port of the CCR2004 is plugged to a unifi UXG which is interfaced to a unifi switch powering unifi APs. Beside that there is an OOB switch (CRS326) wh has its own connection. Power redundancy to the unifidevices is provided by a unifi RPS

What I'm thinking to do is replacing the unifi part by 2 RB5009UPr+S+IN , 1 acting as a backup. The RB5009UPr+S+IN would power the APS (i only have 3) and provide routing. i ill use 2 others one to pprovide some test setup and improve OOB. so for in a line. but my question is how to plug the rb5009 so they get redudant power? Ideally it would be plugged to the 2 UPS. But I can also manage to get an external battery (or 2) id needed Also is there any solution that doesn't require to use that large power boxes like a multi-dc outlet box or something?
Any idea is more than welcome

RB5009UPr+S+IN can be powered by 3 different ways: DC barell jack, 2-pin terminall and PoE-in. However it seems that if PoE-in is active power source, PoE-out is disabled. To me this is kind of expected, PoE-in is quite limited as per standard and power class (up to 30W) and is thus very questionable if device could power all power clients with that limited power budget.
The other two pwer inputs are used in active/standby manner, if both are available then power is drawn from source with higher voltage. If one fails, then power draw is automatically moved to the remaining source. Keep in mind that to keep powering PoE clients, both power sources have to provide voltages suitable for PoE clients (e.g. if PoE clients are 802.3 af/at, then both power sources have to provide voltages in range 48V - 57V).

RB5009UPr+S+IN can be powered by 3 different ways: DC barell jack, 2-pin terminall and PoE-in. However it seems that if PoE-in is active power source, PoE-out is disabled.

I will test it but did you have a source for this. This is very interresting

RB5009UPr+S+IN can be powered by 3 different ways: DC barell jack, 2-pin terminall and PoE-in. However it seems that if PoE-in is active power source, PoE-out is disabled.

I will test it but did you have a source for this. This is very interresting

No, I don't have source for this, it's a collected collective knowledge ...