ho fatto come dici ma dopo aver messo MTU 1500 il test di speedguide mi da questo risultato


TCP options string = 020405960103030201010402
MTU = 1470
MTU is not fully optimized for broadband. Consider increasing your MTU to 1500 for better throughput. If you are using a router, it could be limiting your MTU regardless of Registry settings.
MSS = 1430
Maximum useful data in each packet = 1430, which equals MSS.
Default TCP Receive Window (RWIN) = 257400
RWIN Scaling (RFC1323) = 2 bits (scale factor of 4)
Unscaled TCP Receive Window = 64350

RWIN is a multiple of MSS
Other RWIN values that might work well with your current MTU/MSS:
503360 (MSS x 44 * scale factor of 8)
251680 (MSS x 44 * scale factor of 4)
125840 (MSS x 44 * scale factor of 2)
62920 (MSS x 44)
bandwidth * delay product (Note this is not a speed test):

Your TCP Window limits you to: 10296 kbps (1287 KBytes/s) @ 200ms
Your TCP Window limits you to: 4118 kbps (515 KBytes/s) @ 500ms
MTU Discovery (RFC1191) = ON
Time to live left = 55 hops
TTL value is ok.
Timestamps (RFC1323) = OFF
Selective Acknowledgements (RFC2018) = ON
IP type of service field (RFC1349) = 00000000 (0)

se spunto ppoe, MTU si blocca a 1492
se ti può servire sto usando un modem trust md-3100