I may have to invest in Nvidia Shield as I don't think the shield is limited to 100mbps ethernet connection, I haven't bought it as it came out back in 2015 I think and hasn't been updated since (maybe they were ahead of their competition) but thats what stopped me from buying it as I assumed 2022 Firestick be better as its newer never knew it be limited to 100mbps on ethernet.
Nvidia Shield receives regular software updates.
Have you rebooted firestick and router by switching both off at the main and cleared the cache on the iptv app on your stick? Should run 4k streams fine with 100mbps. Wouldn't bother trying another provider unless lots of other customers are reporting buffering at the same time and on the same channels as you.