iMAC internet speed capped at 100mbps

Still doesn't work; if I change the network hardware to 1000baseTX it disconnects my internet. My iMac is a 27 inch all-in-one, manufactured in late 2015. My Orbi Mesh Router is plugged into the cable modem and then connected to a Netgear ProStat 16 port 10/100 Switch into which the MAC ethernet is connected. I hadn't noticed the switch name previously and it sounds like the switch is the source of my problem; do you agree? If so, I'll buy a faster ethernet splitter (I notice that now all switches on Amazon have gigabyte speeds) . The WiFi operates at over 200 mbps but I'm not sure which device runs WiFi; is it the router? I'm guessing I can check if the iMac is capable of over 200 mbps by simply connecting directly to the cable modem. If so, and the ethernet switch is too slow, this would explain what is happening. I've also upgraded all ethernet cables to CAT 7 because I had such a random mix of cable types and all were much longer than I needed.

I tried your suggestion and it did nothing; I still got the same result. I think I need to figure out how to actually reset my mesh router (it's an Orbi RBR50 with one satellite). The router just seems to pick up where it left off when plugged back in; if it were resetting in some way, I would see a series of lights on top. Interestingly enough, although direct connection to the internet is limited to slightly less than 100mbps, WiFi runs at well over 200 mbps. According to Charter, the problem lies with my computer but Apple says the iMAC should support 200 mbps and your statement backs that up. I'm mystified.

Thank you for all your help. We were both correct. Once I installed a high speed switch and moved my devices capable of using that speed to it, the iMAC automatically adjusted to the same speed as the incoming signal--226 mbps. Before, the ethernet cable ran from cable modem to router to 10/100 ethernet splitter and then to the iMAC. Now, the cable runs from the ethernet cable modem to the router to the gigabyte ethernet splitter to iMAC. The gigabyte splitter also runs to my 10/100 ethernet splitter where I left the original slow speed devices connected. Everything works great now!

For others reading this, the method I used to reset the system was the following: power down the iMAC, unplug the power to the cable modem, and then unplug the power to the router. After the rewiring was done, I plugged in the cable modem and waited two minutes for everything to stabilize, then plugged in the router and waited two minutes for everything to stabilize, started up my iMAC and ran the speed test and the network (preferences) had reset to the faster speed of 226mbps. It took a bit longer for my Arlo cameras to come back on line but everything else came up right away. One cool thing about my new splitter is that lights next to the ports indicate what speed that port is using.