Speeds extremely slow when using router

I've had T-Mobile 5G internet for over a year, and up until now it's been fine. However, over the last few days, I've been getting download speeds < 1 Mbps. It's not a connection issue because when I connect my laptop directly to the gateway, I get nearly 100 Mbps. I've tried two different routers with the same issue. I had been using this same router (D-Link DIR-655) with Comcast for years with no issues. This has happened to me before about every few months, and rebooting the router and/or gateway always fixed the problem up until now. Is this a setting in my router? I know my router is older and doesn't support ipv6 passthrough; could it be something with the 464 translation on T-Mobile's end?