If I understand that correctly,
@sand_man won't see an improvement if the MTU setting is changed in the ONT, as it's just an iperf error as it's reporting wrong mtu so if he uses -l then it's ok = test passed.
@sand_man do -l 1400 -b 180M
Microsoft Windows [Version 10.0.17763.379]
(c) 2018 Microsoft Corporation. All rights reserved.
C:\WINDOWS\system32>iperf3.exe -R -u -b 180M -l 1400 -p 17001 -c queen.cisp.co.za
warning: Ignoring nonsense TCP MSS 0
Connecting to host queen.cisp.co.za, port 17001
Reverse mode, remote host queen.cisp.co.za is sending
[ 5] local 192.168.1.147 port 57977 connected to 62.233.65.195 port 17001
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 5] 0.00-1.00 sec 21.5 MBytes 181 Mbits/sec 0.039 ms 567/16689 (3.4%)
[ 5] 1.00-2.00 sec 21.4 MBytes 180 Mbits/sec 0.022 ms 18/16071 (0.11%)
[ 5] 2.00-3.00 sec 21.5 MBytes 180 Mbits/sec 0.024 ms 0/16074 (0%)
[ 5] 3.00-4.00 sec 21.5 MBytes 180 Mbits/sec 0.032 ms 0/16069 (0%)
[ 5] 4.00-5.00 sec 21.5 MBytes 180 Mbits/sec 0.019 ms 0/16071 (0%)
[ 5] 5.00-6.00 sec 21.5 MBytes 180 Mbits/sec 0.031 ms 0/16073 (0%)
[ 5] 6.00-7.00 sec 21.5 MBytes 180 Mbits/sec 0.031 ms 0/16072 (0%)
[ 5] 7.00-8.00 sec 21.5 MBytes 180 Mbits/sec 0.031 ms 0/16070 (0%)
[ 5] 8.00-9.00 sec 21.5 MBytes 180 Mbits/sec 0.028 ms 0/16072 (0%)
[ 5] 9.00-10.00 sec 21.5 MBytes 180 Mbits/sec 0.032 ms 0/16071 (0%)
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 5] 0.00-10.20 sec 219 MBytes 180 Mbits/sec 0.000 ms 0/163935 (0%) sender
[SUM] 0.0-10.2 sec 22 datagrams received out-of-order
[ 5] 0.00-10.00 sec 215 MBytes 180 Mbits/sec 0.032 ms 585/161332 (0.36%) receiver
iperf Done.
C:\WINDOWS\system32>