Sudden slow wifi

Riccard_2393905 Why would you need two unlimited data SIMs in one household?

If it's because one router doesn't cover the whole property, that's a rather expensive way of going about solving that problem.. you could just have paid for one unlimited data SIM and one router, plus a wi-fi extender device.

Anyway, Smarty's network will not have reduced the speed because you have two SIMs in the same household.. the network can't even tell that both SIMs are in the same household.

A more likely explanation is that both routers are using the same wireless channel and interfering with each other, thereby reducing the speeds attainable. So see if that's the case, and if so, put one of them on a different wireless channel (one that isn't already in use by neighbouring wi-fi networks).

Alternatively, there could just be a temporary network issue in the area.