Cellular ISP with max MTU size 1400 - throughput issues

[gman]

Hi,

My ISP is cellular 5G IPv6 only network (IPv4 address is shared with many users 464XLAT / Carrier Grade NAT). After installing Softether VPN on a VPS (server) and my client on the said cellular network I am facing a lot of speed issues.

For context:

My VPN server (VPS):
- physical location: same city datacenter
- latency: 30ms to 70ms (variable due to cellular)

speedtest.net / fast.com tests without VPN on cellular 5G home internet:
- download: 120mbps - 80Mbps
- upload: 35-60Mbps

With softether client connected via port 443 HTTPS:
- download: 4mbps-5mbps
- upload: 1mbps (sometimes it doesn't work)

My ISP also throttles traffic heavily, OpenVPN UDP with mssfix 1300 is able to get speeds of:
- download: 12mbps - 18mbps
- upload: 8mbps - 10mbps

ShadowsocksR with obfuscation settings running on port 443 I get maximum speeds, no QoS from cellular ISP:
- download: 110mbps
- upload: 50mbps

I believe that softether server and client both try to use maximum MTU size for packets, OpenVPN has settings such as MSSFIX and FRAGMNENT that can be used to control or reduce MTU size used between tunnel connections.

How do you change MTU setting in Softether VPN? Are there other speed or settings tweaks to try? Even OpenVPN (native) runs faster than softether.