-My mother lives in a rural place and uses Ziply, speeds are very consistent at around 2.4/.61 for up/down.
-On her phone (using the nearest t mobile tower, about 3 miles away though no LOS as lots of trees) she frequently gets 22+/1. (Sometimes download is as low as 1.xx but usually is well over 20, or 10-15 in certain rooms)
-Goal: wifi in the home of at least 20 down and 3 up, probably using a T mobile Magenta Max plan as they have unlimited 4g and clearly decent coverage for this area (at $85 per month).
From reading this forum, I've gathered this is what we need to do:
-Order a router and a modem (and possibly some wires - but definitely select one that has a guide on this website)
-Order the Magenta Max sim and use an old smartphone to connect to the router/modem
-Use the guides on this website to setup the phone so that it can connect to the wifi router without thinking it's hotspotting or connected to a router.
-If speeds are still insufficient, we can add an antenna (and probably have to get a weatherproof box for the modem to go outside)
Is that about the size of it?? In addition, I have some questions, if someone could please answer them it'd really help:
1) Upload speeds - Because a router should (I imagine) have a stronger internal antenna than my mother's phone, is it possible to expect higher upload speeds? And if the router doesn't cut it, would an antenna help with upload speeds?
I understand that upload speed is affected by your ISP and not necessarily your equipment, but T Mobile says on its website (for 4G LTE Network On Device): Upload Speed: Upload Speed: Typically between 4 – 20 Mbps (https://www.t-mobile.com/responsibility ... et-service) (it also says for 4g tethering / hotspotting it should be Upload Speed: Typically between 3 – 17 Mbps)
Technically, this will only matter when I'm visiting and want to work remotely but I just want to be on the safe side here!

2) Equipment recommendations - I almost hesitate to ask this because this is frequently asked! Nevertheless, considering our relatively modest goals, I'm wondering how 'strong' we need to go?
I mean on the one hand, my mother is apparently getting quite decent 4g lte download speeds on her phone, so I should expect the sort of lower tier setup like this one to be sufficient:
https://thewirelesshaven.com/shop/routers/lte-rou ... fi-router/
https://thewirelesshaven.com/shop/modems/quectel- ... l-ec25-af/
(as detailed in https://wirelessjoint.com/viewtopic.php?f=8&t=2787)
But if y'all would think it safer to splurge on equipment that is compatible with 5g in order to potentially get better upload speeds (for 5g devices T mobile claims 10 - 34 upload), then well those may be considered of course. Though that equipment is just so much more expensive, so it'd be nice to get by with less if possible.

Thank you for everyone's help! All of this is a lot to take in at first but thanks to all the detailed answers I'm slowly starting to understand everything
