Poor Wireguard throughput over 2degrees/Warehouse Mobile mobile network

I've been observing for a few weeks, but I'm now convinced something changed on 2degrees/Warehouse Mobile. It started when streaming music from my home server over Wireguard started dropping every few seconds. It was fine a couple months ago though, music would stream fine mostly, except the occasional brief drop a couple times an hour perhaps. It's has gotten so bad now that I can't even login my router's web UI over Wireguard due to connections dropping. And forget watching the IP cameras, I'd be lucky if I can even hear a sound coming through.

The problem seems to be limited to Wireguard however, other types connections seem to be fine, e.g. general web browsing and YouTube. Not saying the speed is great (Auckland, pretty much anywhere), but usable in general. I've also tried to isolate the problem, things I've tried:

Accessing my home network over Wireguard from work WiFi, it's fine and very fast. This applies to both a PC and the same mobile phone, in fact I can even saturate my gigabit fibre at home from work.
Back to mobile network, accessing home-hosted web services over HTTPS and that's also fine.
Routing all my traffic over Wireguard to my home network as my gateway and do a test in the Speedtest.net app on my phone, the speeds seem to hover around 1Mbps, but the test fails due to dropped connections I believe.

Perhaps something about handling UDP connections has changed on 2degrees/Warehouse Mobile? Has anyone experienced anything similar?

$ ping -M do -s 1300 server_ip -c 1

PING server_ip (server_ip) 1300(1328) bytes of data.

ping: sendmsg: Message too long

$ ping -M do -s 1252 server_ip -c 1

PING server_ip (server_ip) 1252(1280) bytes of data.

1260 bytes from server_ip: icmp_seq=1 ttl=63 time=160 ms

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siyuan