sagemcom fast 5688w internet going out or losing connection

I've been experiencing connection drops similar to those described here and have exhausted all potential remedies assuming T-Mobile doesn't come up with a fix. Background/notes:

• T-Mobile Home Internet customer for a year. Was issued Nokia gray cylindrical gateway (aka trashcan). Steady connection averaging 300Mbps download.
• Mid-November, connection began to drop. Worked great during the day but cut out at night. Since the Nokia wouldn't reacquire the signal, the only remedy was to restart (turn it off and then back on). Once back on, performance ok until the next evening.
• After a week (late November), called customer support. Rep mentioned signal congestion but confirmed no tower maintenance or issues in my area. Suggested I swap the Nokia for a Sagemcom Fast 5688W at a T-M store. Did so.
• After three days (early December), 5688 connection dropped each day for three consecutive days. Only remedy was a restart each time. Prior to drops, signal strength was very good to excellent and average download speed was 350Mbps. Called customer support, new 5688 shipped overnight.
• After four days, new 5688 connection dropped each day for the first two (Dec 11, 12) and four times on Dec 13. Only remedy was a restart each time. After the second drop, called customer support, no solution to address the issue. After third drop, took gateway to local store. Rep swapped the SIM card. Back at home, no issues until the fourth drop. Called customer support, was instructed to confirm the SIM and IMEI numbers, unplug and leave the 5688 off while she performed a signal refresh, and turn it back on. She created a ticket for T-M engineers to address. No issues <24 hours since speaking with her and posting this.

In all cases:

• Each gateway was placed in the optimal location confirmed by the app’s placement assistant
• Signal strength was very good/excellent before connection drop and immediately after restart
• Each gateway was unable to reacquire the lost signal
• No tower upgrades/maintenance underway

In some cases, the message on the gateway read "No internet connection". In others (specific to both 5688s), the following message was displayed (word for word): "Attention, Internet connection issue. Press power button to turn off device for 10 seconds. If the issues persists, contact Customer Support. [Error Code WAN002]. No rep at T-M has been able to explain exactly what Error Code WAN002 is.

So, after a year of steady, no-issue service for $50/month, three gateways (Nokia, Sagemcom Fast 5688W, a second 5688W), four calls to T-M customer support, and two visits to a local store in the past three weeks. And, no one at T-M can explain the issue despite restarts, SIM card swaps, IMEI checks, and location clearances. Espléndido.