Need Cheap 3G/4G Remote Router Suggestion

We have been using the TP-link mr3220 router along with a 3G cellular USB modem to connect to our remote solar installations to monitor performance of our PowerOne Aurora and SolarEdge inverters. The advantage was that this router was cheap (~$35) and very reliable.
My problem is that they no longer make it (hardware verison 1.2) and its successor (version 2) is not reliable and is not fully supported by custom firmware. I can't even find them on ebay anymore.
Does anyone have a suggestion for a RELIABLE router alternative (i.e. works for over a year with no user intervention) and cheap?
Comments
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,
2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.
For cheap, the suggestion of something like a Linksys wrt54 reflashed with dd-wrt/tomato/whatever is likely to be pretty reliable. Something like a Raspberry Pi with a stripped down *nix OS might work well too.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
http://members.sti.net/offgridsolar/
E-mail [email protected]
TP-Link has small newer USB units like the TL-WR902AC but it won't stay online for more than a few days with the cellular stick. Anything under $100 really would do the job, it has to have a USB port with 3G/4G support, stay connected, and be supported by DDWRT
I've used this watchdog approach with an automation PC that I needed to keep on the Internet for weeks at a time, but I had little confidence in the router.
750w:100Ah LFP
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter