RAX48 hacked?

Not being offered a firmware upgrade on the webinterface/ mobile app immediately, although you can find it for manual downloading/ installing, wouldn't worry me too much, as the world-wide deployment seems to happen slightly gradually. Obviously that shouldn't leave you more than one firmware version behind - and that only for a limited amount of time, until the automatic deployment reaches your device.

(Disclaimer: I don't know that exact device, nor which initial firmware version it ships with, which would be the basis to know if you should get offered an upgrade immediately (because the current version has been around for longer) or if it might just be a matter of time).

While any technical device poses the risk of being hacked due to security issues, I wouldn't quite expect that to happen this quickly, nor visibly - after all 'professional' attackers are more interested to remain under the radar (adding your network to their botnet, injecting ads, seeking weaknesses in your LAN and potentially staging extortion attack, etc. pp.), rather than forcing you off the network and thereby making you notice immediately (and sort out the issue/ pushing them out again) . Although it's very hard to guess, based on the provided information, it's imho more likely that you've fallen prey to accidental misconfiguration (think auto-correct messing up SSID/ PSK or one of your family members having done a mistake). The alternative would be that somehow your configuration was left open wide enough, to allow rather unskilled script-kiddies to get access (things like PSK/ admin password way to weak, wireless encryption intentionally disabled, remote configuration with way to weak passwords). In general the default configuration wouldn't really pose that risk (although you should definitively pick a custom ESSID and add better/ strong passwords/ PSKs). If you suspect foul play, doing a full factory reset would also be strongly advised.