In the meantime, I can add that 5G also works in the Sunrise network if you replace the installed Android with /e/ OS. However, there is no speed advantage with my subscription (swype). Technically, 4G is good for up to 1 Gbit, if I understand it correctly. Only very few mobile contracts with the label "5G" promise transmission rates of more than 200 Mbit (of course only in the best case). In this respect, 5G is probably more of a marketing stunt so far...

The Fairphone only supports the NSA (non-standalone) mode of 5G. However, since Sunrise itself also sells devices that can only support NSA, this cannot be the reason. Unfortunately, I have not been able to find any other specifications (at Sunrise). This should be a case for contacting the provider.
I also have the FP4, but 5G is currently uninteresting for me as long as it generates additional costs - that's why I didn't research this before buying.

I've had the Fairphone 4 for a few days and I haven't had any changeover either. 5G is used on the Siwsscom network wherever available. That was one of the reasons for replacing my 5-year-old Fairphone 2. I am very happy with the new device so far.

I have the Fairphone 4 and am on the Swisscom network with 5G. I didn't need to switch, it just worked like that.