RUT240 set-up should be no different to RUT230 as they both use the same firmware. Please follow these options and see if there's anything you might've missed.
1. Update your router to the latest firmware available.
2. See if you haven't locked specific frequency bands or network type, this might cause issues if the reception isn't great in your area.
3. Check the cables and antennas if they're alright and connected tightly.
4. Try resetting your router to default factory settings.
If nothing from above works, I would kindly ask you if you could share the configuration that you've used on RUT230 and using now on RUT240. Also, add screenshots of the exact issues that you're facing.
Regarding external antennas for mobile, if your signal is alright already, I would suggest using the antennas that you should've got with the device itself. If you're somewhere where the signal isn't great and you have a possibility to mount antenna on the roof or some kind of placeholder, I would suggest using outdoor antenna: https://teltonika-networks.com/lt/product/combo-mimo-mobile-roof-sma-antenna/
To give you better recommendation I would need to know the details about the area and the situation with cell towers around, but I believe this antenna should be enough if you didn't have issues with RUT230.