I am trying out a new internet provider (T-Mobile 5G Access Point) and so far it’s far better than my current provider (Centurylink DSL). Since I’m stating with a blank slate on that front, what is the best configuration? Do I want a separate network for Home Assistant and it’s devices? Assuming I don’t use the same SSID and password, what is the best way to transfer all my devices? The Tasmota, Tuya and ESPHome devices are my biggest concern. I have to do all them before I move the HA box right? Any advice from someone who has done this? Thanks in advance.
I don’t. Why complicate things?
How is it better, except that DSL is 1980 technology. (I didn’t think anyone was still using it).
What kind of speeds do you get with speedtest, and how does it compare to what you were promised?
If you can (have the proper hardware and skills) that’s typically what one could call “best practice”.
Well, for ESPHome you don’t need any concerns. Just did a seemless wifi swap of almost 100 esphome nodes and it just worked 
In detail you can have a esphome node configured to make use of more than one wifi network:
# Example configuration entry
wifi:
networks:
- ssid: FirstNetworkToConnectTo
password: VerySafePassword
- ssid: SecondNetworkToConnectTo
password: VerySafePassword
# Other options
# ...
just don’t forget to ota that config to all your devices while the old wifi is still active 
Best practice is considered to be a segmented network, but it requires alot more administration, since you will need to punch some holes in the wall between them to get everything up and running.
Regarding the move, then it sounds like you have a pretty decent access to the router setup, so if you can then set the router up with the same settings as the old one, like default gateway, IP range, static leases, but also SSID and passphrase for the WiFi.
When that is done, then moving the devices should be a cake walk.
The T-Mobile internet is 10-20x faster than DSL. I live in a rural area, so the new stuff takes a while to get here.
That’s what I was looking for! I didn’t realize you could configure for both!
Everything is faster than DSL- what speeds are you getting from speedtest?
DSL covers a lot of technologies and the newer ones can deliver speeds up to 100 Mbit/s, which in many cases might be the best option.
Not all places have access to fiber or coax and then DSL might be the best choice.
A 5G mobile data plan might be good at the moment, but it could change over the next year or two with more and more getting mobile phones that will use that frequency band.
5G is a protocol- not a frequency.
Cellular carriers my use any protocol on any frequency they have licensed.
This is why some carriers are “turning off” 3G service. They want to repurpose the frequencies for 5G. In most cities they are putting up 5GHz nodes for 5G service, which is probably why many people confuse the 5G protocol with 5GHz. In some very densely populated cities, like Manhattan, they are using the 60GHz band inside of buildings and tunnels. (60GHz will barely penetrate walls, let alone tunnels). In sparsely populated areas 5G, 4G nd 3G can coexist on the same frequency band.
The prevalent means of connecting to the Internet in Germany is DSL, introduced by Deutsche Telekom in 1999
True, but 5G is using a licensed frequency decided by the RF governing body in the area, so 5G is tied to a frequency.
Its also true that Cellular providers are turning off 3G to use it for 5G, but to fulfill the promises made with 5G, like using it as an alternative to cabled connection, you need the higher frequency bands, and when its used as an alternative for a cabled connection, then the bandwidth usage at the cellular providers will increase.
The old 3G frequency is mostly aimed as 5G for IoT cases, where the lower bandwidth is not an issue, but the long range and therefore connection stability is desired. especially for moving devices.
Globally, more people are still served by DSL family of technologies than fiber optics / cable.