We had a similar problem with the PTP 550 (brother to ePMP 3000/300). We got them in last week and and did a quick set up using a SOHO router. Setup was like this Fiber>Mikrotik>Master<>Slave>SOHO>devices. Devices wired to SOHO had good speed tests and latency (in relation to router we were using). Devices connecting to SOHO wireless had slow speeds (around 15Mbps up and down) and ping times from laptop to master varied from 3ms to over 200ms. SOHO was on 2.4 band so no self interference. Threw a cheap 1 gig Netgear switch between slave and SOHO and issue went away.
I am not a networking guru (we contract the heavy networking for now) so I only have a good guess to why this is. I am sure someone like Chris Bay, Newkirk, and others could probably tell you down to the packet what the issue is/was.
Not too concerned, the 550's seem to work fine when connected directly to Mikrotik routers, which is how they will be ran when released into the wild. For anyone wondering, when set up using tik routers, using 20Mhz+20Mhz channel bonding, we were seeing speed tests (AT&T speedtest) of 240+Mbps down and 60+Mbps up (MCS9 all around). On other speed test sites we were getting 150-200+Mbps down. Pretty good for a single pc speed test.
I adjusted the mtu manually on the end point computer and still had the problem. I'm not surewhat mmclamping is.....I've never seen those settings. One router is a R201P and the other an EDI Max AC1200....
As much as I love about PPPoE I really hate the MTU issues that crop up from time to time these days. We used it for many years without a problem really but in the last 3 years it seems like we are constantly dealing with some new problem (it always breaks the streaming services ... always) caused either by something the CDNs changed or Miktrotik and Cambium updates breaking stuff.