Isn't this similar to the pre-iPhone days, when if you brought an overseas GSM phone you had to go through some hoops to get it activated with Cingular/T-Mobile? I remember getting handsets from China and Europe and the reseller would have to circumvent something to get it activated on the networks.
On the other hand, some handsets that are blatantly missing LTE bands are on the whitelist, such as the Chinese Oneplus 8T.
To be frank: No. You popped your SIM in and the phone worked immediately on the network. If it didn't work you manually entered APN settings in.
Some years later AT&T started maintaining a whitelist of smartphone IMEI numbers so they could charge more for data plans than a featurephone would pay. T-Mobile did not do this. AT&T went as far as maintaining whitelisting for phones that would be allowed on wifi calling or voLTE, T-Mobile phones just needed to carry the profiles.
No 'circumvention' other than carrier/subsidy unlock was ever required to use an international phone save for the very rare times international phones had bands software disabled (and this is quite rare).