I'm still on a circa-2015 data plan so I believe my international data is capped at 256kbps and it works surprisingly well. I use Opera Mini for web browsing on it and WhatsApp w/Use Less Data for Calls for voice. I also have background data transfers disabled on the phone. It honestly works a lot better than I'd expect it to.

frappant I don't know if this will make a difference in your case, but I've always rebooted the phone when I get to a new country. Cycling airplane mode may do the same thing, but I know a reboot always has worked for me.