That is quite interesting! Except for the interval 1s-4s, in which the throughput collapsed to zero and then rebounded, your phone tethered Gargoyle router was delivering a fairly consistent 90+Mbps to your laptop, which is a bit better than I expected. I don't know enough about Wifi to theorise why it stalled early on though
I think this is enough evidence to suggest that the major performance bottleneck is more likely than not upstream of Gargoyle, and the fact that TTL adjustments seem to help - even if only modestly - also seems to me to point in that direction.
Other than looking for alternative phone plans/SIMs available to you that have less restrictions on how data is used, I can't think of anything else to pursue.
BTW: if your iperf3 client supports all the standard iperf3 options, the
-R (or
--reverse) option would allow you to test throughput in the upload direction which might also be worth knowing.