Help to repair, upgrade or replace 5-1/2 year old residential gateway?

My AT&T U-verse Residential Gateway (RG) equipment, Pace Plc Model 3801HGV, has been active in my home since September, 2013, (over 5-1/2 years).
The AT&T-provided UPS has been in "alarm" mode for over a year, (it's been switched to Silent.)
The RG reboots almost daily, now, though electricity in my suburban St. Louis home is reliable & stable.
I continue to pay for 25Mbps, but Internet download speeds have fallen from 20Mbps to 2Mbps or less.
Trying to troubleshoot, I've disconnected all wired & wireless devices trying to eliminate causes, but slow speeds persist on even just one test device.
From a new iPad, I've used speed tests like DSLReports, Ookla and Netflix FAST to measure down/upload speeds.
They all show consistently 2Mbps or less download and similar upload.
I would request a service call from AT&T, but I don't subscribe to "wire service", and I don't think I should have to pay for a service call on equipment I rent from AT&T.

Any help or guidance is appreciated.