RUTX12 Modbus problem: "ERROR: read register failure - Connection reset by peer"

Hi. I have a problem with RUTX12 routers. I am requesting following data:
System uptime
Mobile signal strength
System temperature
GSM operator name
Router serial number
Network type
Mobile data received last 24h (SIM1) + Mobile data sent last 24h (SIM1)
Modem ID
GPS latitude coordinate + GPS longitude coordinate
All are requested in 1 min inverval except for serial number which is requested once every 24h. On top of that every request is triggered in sequence once after another in 0.5 s interval. I also change modem every 1 min to get data for both SIM cards by using linux commands:
“printf “\x00\x00\x00\x00\x00\x13\x01\x10\x01\x48\x00\x06\x0C3-1\x00\x00\x00\x00\x00\x00\x00\x00\x00” | nc 502 | od -An -tx1”
“printf “\x00\x00\x00\x00\x00\x13\x01\x10\x01\x48\x00\x06\x0C1-1.2\x00\x00\x00\x00\x00\x00\x00” | nc -w 5 502 | od -An -tx1”
Everything works fine for some time but then it randomly stops and I get error in modbus response for every request:
“ERROR: read register failure - Connection reset by peer”

And error when executing modem change:
“read(net): Connection reset by peer”
What can be the cause for that? Modbus is working of default settings and ports. The is one router that doesn’t have this problem and one where I switched modem changing off and it works ok so far so I suspect it might be related to switching modems.
Rebooting router restores Modbus responses for random time again.

Could really use help with this one, I wasn’t able to find any solution to it. With switching disabled the routers and modbus work without problems but I need to monitor SIM2 data somehow.