In some programs that are used for reading the DSMR meter, it is assumed that at least two UARTs are available. Unlike ESP32 and related boards, the WeMos D1 does not have a dual UART on board. The second UART on the ESP8266 is used for flashing the eeprom. Therefore only Serial can be used in the sketch and not Serial1. That makes using the WeMos for reading the DSMR meter complex.
My advise check this regarding the use of Serial and Serial1 – Reference · ESP8266 Arduino Core

Basically, if you are using Serial, that is the connection on UART0 RX and UART0 TX.
This is the connection that is already connected on the board to the USB to Serial bridge.
As a result, if you are using Serial, that should be stuff you are either reading from or writing to the computer. Hooking anything else to the UART0 RX and TX lines is strongly not recommended despite how much you might think it would simplify your life as it is going to result in logic level conflicts, potential damage to chips and (more likely than not) not getting the results you expect.

The only exception to this is if you are both sure that the USB to serial bridge tri-states the lines (i.e. goes high impedance) when the USB connection is not active and you also are not intending to communicate the data to the computer at all – e.g. you are displaying it locally on an LCD, passing data over Wi-Fi / Bluetooth (OTA) or storing it to a microSD card etc.

If you are using Serial1, then it is using UART1 RX1 and UART1 TX1.

Yes, there are two hardware serial ports on the ESP8266 and this one uses different pins for connection, but this second UART comes with a major issue –
you cannot use it for receiving data because the RX1 line is shared with the flash chip that is providing the data for the sketch itself!!!

Attempting to use it will (you guessed it) cause logic level conflicts and potentially even sketch crashes as the core fails to read from the serial flash memory. This is just how the ESP8266 “package” is, unfortunately, and the ESP32 is a bit more forgiving in this regard (to what I know).

This is why I have recommended set-up software-serial to have a second software-based UART on any set of pins you would like. This is not 100% perfect especially at higher baud rates or if you’re doing a lot of work inside the sketch as it does consume some CPU time and comes with risks of losing data if the timings are unfavorable.

ESP8266 GPIO Blocked use