I am using a technicolor tc4400-am modem hooked up to a linksys router and a raspberry pi 4 model b running openwrt. I made the WAN port's MAC address on the pi the same as the linksys router's WAN MAC address. The problem is that when I am using the pi there are connection issues on the linksys router. For example if I run a speed test on both routers at the same time one of them will lag until the other is done. If I change the MAC address of the pi then my ISP comes in the way by blocking that MAC address. Is there anything I can do about this? All your suggestions are appreciated. Thanks
It's not quite clear from your description how these devices are connected to each other.
Based on your error description (and having looked at pictures of the tc-4400), I fear you're trying to connect both WAN ports (linksys && RPi) to the two LAN side ports of the tc4400 at the same time, that doesn't work. There mustn't ever be duplicate MAC addresses on the same physical subnet (and the two LAN ports of the tc4400 are presumably bridged in its internal switch). Unless your ISP allows two concurrent internet connections over the modem (pretty much all consumer ISPs do not), each with their own -different- MAC address, you can only connect one of your devices to the modem and use that as router (so probably the RPi), the Linksys then needs to go behind the RPi, as dumb-AP; ideally you don't re-use the cloned MAC address either (meaning it's best to change the WAN MAC of the Linksys to something else as well[0]).
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[0] because of this problem, I always use a 'fake' MAC address (technically a real one from a defective device I've disposed years ago) on my WAN, this reduces the risk for accidents (the ISP always gets to see the same, fake MAC - which is different from the real MAC addresses of all of my devices, allowing me to switch out routers (more-) easily).