Chris,
I just did set-up a wemos on my Arduino to test you issues.
I did set-up in control panel as you can see in attachement
and after connection all went well as per attachement
So I did nothing elese tha Flashing the Wemos, connecting it to the Arduino Mega
Wemos Arduino
TX ===> RX1
TR ===> TX1
5V ===> 5V
Gnd ===> Gnd
and heunder the logs of the connection
Are you sure you did setup the right port in OnStep?
============ log under /home/alain/.indi/logs/2018-12-01/indi_lx200_OnStep/
INFO 0.002935 sec : Session log file /home/alain/.indi/logs/2018-12-01/indi_lx200_OnStep/indi_lx200_OnStep_17:16:01.log
DEBUG 0.006140 sec : Active Snoop, driver: Dome Simulator, property: DOME_PARK
DEBUG 0.006761 sec : Active Snoop, driver: Dome Simulator, property: DOME_PARK
INFO 19.362766 sec : Connecting to 192.168.0.1@9999 ...
DEBUG 19.364743 sec : Connection successful, attempting handshake...
DEBUG 19.364871 sec : Testing telescope connection using ACK...
DEBUG 19.368476 sec : Testing successful!
INFO 19.368587 sec : LX200 OnStep is online.
DEBUG 19.372537 sec : Configuration successfully saved for CONNECTION_MODE.
SCOPE 19.373428 sec : CMD <:GR#>
SCOPE 19.380086 sec : RES <11:42:58>
SCOPE 19.380134 sec : Coordinate format is high precision.
SCOPE 19.380148 sec : <ACK>
SCOPE 19.380159 sec : CMD <0X6>
SCOPE 19.383983 sec : RES <P>
SCOPE 19.384026 sec : <getTimeFormat>
SCOPE 19.384041 sec : CMD <:Gc#>
SCOPE 19.389915 sec : RES <24>
SCOPE 19.389973 sec : <getSiteName>
SCOPE 19.389988 sec : CMD <:GM#>
SCOPE 19.397218 sec : RES <Schoenau>
DEBUG 19.397247 sec : Site Name <Schoenau>
SCOPE 19.397272 sec : <getTrackFreq>
SCOPE 19.397281 sec : CMD <:GT#>
SCOPE 19.404310 sec : RES <0.00000#>
SCOPE 19.404400 sec : VAL [0]
SCOPE 19.404421 sec : <getSiteLatitude>
SCOPE 19.404429 sec : CMD <:Gt#>
SCOPE 19.411109 sec : RES <+48*13>
SCOPE 19.411171 sec : VAL [48,13]
SCOPE 19.411181 sec : <getSiteLongitude>
SCOPE 19.411189 sec : CMD <:Gg#>
SCOPE 19.418669 sec : RES <-007*39>
SCOPE 19.418720 sec : VAL [-7,39]
DEBUG 19.418738 sec : Mount Controller Latitude: 48.2167 Longitude: 7.65
DEBUG 19.419281 sec : Configuration successfully saved for GEOGRAPHIC_COORD.
SCOPE 19.419351 sec : CMD <:GG#>
SCOPE 19.424602 sec : RES <-01>
SCOPE 19.424672 sec : VAL [-1]
SCOPE 19.424701 sec : CMD <:GL#>
SCOPE 19.432255 sec : RES <13:31:06>
SCOPE 19.432319 sec : VAL [13.5183]
SCOPE 19.432338 sec : <getCalendarDate>
SCOPE 19.432349 sec : CMD <:GC#>
SCOPE 19.440733 sec : RES <12/01/18>
DEBUG 19.440819 sec : Mount controller UTC Time: 2018-12-01T11:31:06
DEBUG 19.440835 sec : Mount controller UTC Offset: 1.00
SCOPE 19.440860 sec : CMD <:GVD#>
SCOPE 19.449614 sec : RES <Dec 1 2018>
SCOPE 19.449693 sec : CMD <:GVT#>
SCOPE 19.457610 sec : RES <18:06:16>
SCOPE 19.457676 sec : CMD <:GVN#>
SCOPE 19.465316 sec : RES <1.17b>
SCOPE 19.465370 sec : CMD <:GVP#>
SCOPE 19.471376 sec : RES <On-Step>
INFO 19.471573 sec : Mount is unparked.
DEBUG 19.471766 sec : InitPark Axis1 0.00 Axis2 0.00
INFO 19.471796 sec : =============== Parkdata loaded
SCOPE 19.472040 sec : CMD <:FA#>
SCOPE 19.476162 sec : CMD <:fA#>
INFO 19.481287 sec : Mount is unparked.
DEBUG 19.481473 sec : InitPark Axis1 0.00 Axis2 0.00
DEBUG 19.481502 sec : Setting Default Park Axis1 to 0.00
DEBUG 19.481519 sec : Setting Default Park Axis2 to 48.22
SCOPE 19.482163 sec : CMD <:Sg352:21#>
SCOPE 19.490033 sec : CMD <:Sg352:21#> successful.