If the devices are out of warranty and there is no insurance on the devices to extend warranty then buying new or used devices will be the only option other than finding a friend or relative with used devices that they can give or sell to you.
Now you can go through T-Mobile or a resell website to get new or new to you used devices and choose any price point that is in your budget.
What exactly did you ask T-Mobile to do as far as options go?
Tmobile won't discount any device lower than it's currently advertised price. And other criteria must be met as well.
My ex's (we still are friends) Note 4 started doing the same thing, it was just the battery. Lithium batteries are not lifetime batteries and will wear out after time (charge cycles) and this usually isn't a slow degradation process but typically happens fairly rapidly when they start to go. Just got her a battery at batteries plus and it resolved most her issues. Eventually made sure her contacts were backed up to the Google cloud and then did a master reset and that resolved most of the other outstanding issues. I think the few remaining issues are more due to that some of the newer apps are tailored to the most current Android OS (Nougat) while the Note 4 is and will remain on Marshmallow and is at least a couple of years old at this point.
As for used phones especially the Notes most are still Note 4's as the Note 5's are harder to get since the Note 7 (they skipped the Note 6 nomenclature) fiasco a lot held on to their 5's so they are harder to come by. Of course while TMO does charge full price (as do all the carriers now) since the "contract" plans went away they do offer interest free financing of those devices which if you add to the current service plans is still less than the old contract plans. However if that is not an option they do offer some some decent devices at the mid and lower end if it turns out the battery is not the issue. At the mid level the Samsung J7 isn't a bad device at all and essentially it is some screen resolution and memory/storage that it sacrifices for price, it is about $250 occasionally less at other retailers that sell T-Mobile prepaid phones. If you are at say Wal-Mart, Target or Best Buy and you see a T-Mobile prepaid phone that is a good price it will work on your post paid plan, you may just have to purchase a new SIM card (the prepaid SIM card can't be used for post paid) since the Note 4 one may or may not fit, if it does then just take it out of the Note 4 and put it in the new device and that's it besides restoring your data.