It depends on the phone. The rule of thumb in the past for typical dual-SIM phones was nG on the first SIM, n-1G on the second. Then, after you encounter that f

It depends on the phone. The rule of thumb in the past for typical dual-SIM phones was nG on the first SIM, n-1G on the second. Then, after you encounter that for the first time, you make sure that your next phone has a baseband that supports nG on both SIMs, although with 5G it's become a bit blurred, typically 5+4 or 4+4, not that that makes any difference (5 vs 4 I mean).