Network coverage specifications were included in the "Astro Slide Campaign - Week 42 Update". It was sent by email as backer update #17 on January 11, and is also available here
https://www.indiegogo.com/projects/astro-slide-5g-transformer/x/16342699#/updates/all In, short, these frequencies and bands are listed as supported:
2G GSM/GPRS/EDGE: 850/900/1800/1900MHz
3G WCDMA/HSPA+/UMTS: B1/2/5/6/8
3G CDMA/CDMA-EVDO1X: BC0(800MHz)/BC1(1900MHz)
4G TDD-LTE: B34/B38/B39/40/41(Full)
4G LTE bands: B1/B2/B3/B4/B5/B7/B8/B12/B17/B18/B19/B20/B26/B38/B39/B40/B41/B66/B71
5G bands: N1/N3/N28/N38/N41/N77/N78/N79
As this list is six months old, you may have to ask Planet if it's still valid.
On its own, that bunch of numbers might not say all that much. These lists might be helpful:
https://en.wikipedia.org/wiki/List_of_5G_NR_networkshttps://en.wikipedia.org/wiki/List_of_LTE_networksThough, as you write that carriers are changing frequencies for the 5G rollout, those lists might not be up to date or may not accurately predict the near future. Perhaps your carrier can tell you which bands they will have support for.
The Gemini supports relatively few bands, so whether it will or won't be supported, might not be a good indicator of Astro support. Overall, the Astro is, according to Planet, supposed to cover enough bands to, provided a valid SIM card, basically work in all countries that has a cellphone network. Then again, I suppose, there is a bit of marketing speak in that statement. Best of luck.
