The router should generally work with a Yallo SIM card, as it is compatible with most SIM cards worldwide. However, you must ensure that the Yallo SIM card can be used in Germany under roaming. You should ask Yallo whether roaming is supported in Germany and whether there are any special settings you need to make. You should also activate the data roaming function in the Deco app to ensure that the connection also works outside the regular network area.