You didnt mention which Ultra model, but if you got it last year its most likely a 4800, which has a better WiFi chipset than previous Ultra models (and supports much higher 5Ghz bonded channel speeds), and assuming you have firmware version 10.5.0-4208, you should see speeds around 200Mbps (assuming a 5Ghz AC connection) when checking the connection via Settings/Network/Check connection.
You say you didnt change anything to your modem and router/gateway; however, that doesnt mean they didnt change themselves (via auto-updating), via both firmware and/or settings updates, especially when managed by Comcast (they control the firmware updates/settings for the modem at least), and possibly the router/gateway too, if its Comcast provided).
In addition to that, the 2.4/5Ghz WiFi channel assignments can/do change as well (if not manually configured), which can result in degraded performance if currently on a highly used channel (nearby/neighbor's devices on the same channel).
1) Make sure you have separate 2.4Ghz and 5Ghz SSIDs, and that channel bonding/bandwidth is set to Auto or 40/80/160.
2) Verify your Ultra is connecting via 5Ghz.
3) Verify the 5Ghz channel isnt overused/overlapped (e.g. use WiFi Analyzer on an Android phone/tablet for this)
4) Manually configure 24/5Ghz channels to unused/underused ones in your modem/router/gateway.
