Orbi RBR 50 router firmware - 2.7.4.24 or 2.7.5.4?

2.7.5 comes after 2.7.4. 

NG sometimes doesn't push or make avialble updates on the RBR so it's up to you if you want to update or not. You don't have to. 

I recommend if you do, manually update as suggested here:

https://community.netgear.com/t5/Orbi-Wi-Fi-5-AC-and-Orbi-with/New-RBR50-RBS50-Firmware-Version-2-7-5-4/td-p/2296598/jump-to/first-unread-message