the option to switch to 3G does not occur in the normal settings choices under advanced.

With Fi selected

You can dial *#*#4636#*#* . Once you've done that you'll see Phone Info.->Set Preferred Network Type: LTE/CDMA/UMTS auto (PRL). This is the default PRL that prioritizes LTE signals 1st, then CDMA(Sprint/USC), then UMTS(T-Mo). It works in conjunction with the dialer codes for the carrier, so it doesn't work completely independently. So if you are on T-MO 4G-LTE and want to switch to 3G, you'd change the PRL to WCDMA preferred, giving you UMTS-voice, and HSPA(3G) data. If you were in T-MO and wanted to switch to Sprint, you'd use the dialer codes already discussed or FiSwitch to select Sprint. Then if you wanted 3G sprint, you'd use this menu to select CDMA auto. The thing to remember is that after altering these settings you should set the PRL back to LTE/CDMA/UMTS and REMEMBER: These changes only last for 2-2.5 hours, just like the carrier dial codes previously mentioned. In addition this dialer code also tells you a lot more about your phone connection, and there is a menu for your wifi connection, like the ip address and mac address.

When I toggle to my T mo data sim in the physical sim slot, the option to change preferred network appears on the normal settings under network & internet >> mobile network 2 sims >> T mobile active sim, scroll down and preferred network type is there and can be changed. I have only had to do this a couple of times.

https://support.google.com/fi/thread/229010?hl=en