The mobile part testing for the RUT240 device is covered with the integrated mobile module (EC25-J) test reports. This is common practice when certifying device with integrated mobile module which has been previously certified in Japan.
The EC25 - J module certificate provides the following antenna specifications:
PR1KS210 is a magnetic type antenna, witch matches the type of construction with the gain of no more than 1dBi, which is below 3dBi, meaning that it complies with the certification. Nothing is needed to be done from the mobile (module) side, and the current EC25-J certificate is valid with the PR1KS210 antenna.
In short, you can use the router and antenna combination in Japan.